Inca Architecture, Engineering and Infrastructure

Inca architecture, engineering, and infrastructure constitute the built legacy of the Sapa Inca and the imperial state that ruled the Andes from the early fifteenth century until the Spanish conquest of 1532. The term encompasses the formal canon of imperial stone buildings, the cosmologically planned layout of cities such as Cusco, the rural terraces of the Sacred Valley, the network of roads and bridges that bound the empire together, and the hydraulic, agricultural, and storage systems that sustained a population of perhaps ten to twelve million people. Inca builders produced monumental temples, fortresses, palaces, shrines, tambos, storehouses, aqueducts, and irrigation canals using techniques that combined careful surveying, dry-stone fitting, and a high degree of standardization. Their construction methods were later studied both for their aesthetic refinement and, especially in the wake of earthquakes in the twentieth and twenty-first centuries, for their remarkable resistance to seismic movement.

The architectural tradition drew on earlier Andean cultures, including Wari, Tiwanaku, Chimú, and the Killke, but the imperial Inca synthesized these elements into a coherent imperial style. This style is recognizable from Quito in the north to Mendoza in the south, and from the eastern flanks of the Andes to the Pacific coast. Although most Inca buildings are modest in elevation, the precision of the ashlar masonry, the integration of structures with natural topography, and the use of earthquake-resilient techniques make the corpus one of the most distinctive in the pre-industrial world.

Overview

Inca architecture is conventionally divided into two broad modes. The first is rustic masonry, in which stones of irregular size are bonded with mud mortar, sometimes faced with clay or cobblestones. This mode was used for vernacular housing, terraces, and most of the rural infrastructure of the empire. The second is ashlar or finely cut polygonal masonry, in which stones are shaped so precisely that they fit together without mortar. This mode is the signature of the imperial style and was generally reserved for temples, royal residences, administrative centers, and the most important ceremonial buildings. The best-known examples are concentrated in Cusco, at Sacsayhuamán, at the Coricancha, and at royal estates such as Machu Picchu, Ollantaytambo, and Choquequirao.

Beyond the masonry itself, Inca engineering is notable for its integration of the built environment with the natural landscape. Cities were sited on commanding promontories and laid out in conformity with cosmological concepts. Hilltops were sculpted into the form of sacred animals, particularly the puma, which became the zoomorphic outline of Cusco following the urban reforms attributed to the ninth Sapa Inca, Pachacuti Inca Yupanqui, in the mid-fifteenth century. Terraces, drainage channels, and platforms were cut into mountain flanks, while isolated standing stones, or huacas, were incorporated into urban and rural landscapes as foci of religious veneration.

Imperial building was carried out largely through the mit’a system, a rotating form of corvée labor by which communities provided workers to the state in exchange for the protection of the Inca and the right to participate in the imperial cult. Engineers, known as camayoc, organized labor, drafted plans, and supervised construction, but the names of few have been preserved in the colonial record. Buildings were typically inaugurated with elaborate ceremonies, the offering of llamas, and the ritual exchange of maize beer, or aqha. Once a complex was completed, it was dedicated to the sun god Inti, to the creator deity Viracocha, or to one of the huacas of the region.

The architectural tradition

Inca architecture is a direct product of the political consolidation of the Andes under the Inca between roughly 1438 and 1532. The dynasty is conventionally divided into a legendary pre-imperial period of early rulers, the reign of Pachacuti Inca Yupanqui (c. 1438–1471/72), and the reigns of his successors Túpac Inca Yupanqui (1471/72–1493), Huayna Capac (1493–1527), and the brief reigns of Huáscar and Atahualpa, who fought a civil war immediately prior to the Spanish conquest. The imperial style is most closely identified with the era of Pachacuti and his immediate successors, when the building program of the state reached its most ambitious expression.

The Inca built upon a long Andean tradition of stone construction. Earlier cultures, including the Wari (c. 600–1000 CE) and Tiwanaku (c. 500–1000 CE), had developed substantial stone and adobe architectural vocabularies, and the Killke culture that preceded the Inca in the Cusco region left visible precedents in fieldstone construction. The Inca absorbed these elements but added two distinctive features: a much higher degree of refinement in the cutting of individual stones, particularly for elite buildings, and a deliberate imperial iconography expressed in the puma, condor, and serpent forms used in urban planning. The puma outline of Cusco is the clearest example, with the fortress of Sacsayhuamán forming the head, the Coricancha marking the navel (the literal meaning of Qosqo in Quechua), and the confluence of the Huatanay and Tullumayo rivers representing the tail.

Inca buildings were not, in most cases, designed by named architects in the modern sense. Designs were attributed to the Sapa Inca himself or to the god Viracocha, who was said to have instructed the first Inca in matters of construction. The chronicler Pedro Sarmiento de Gamboa recorded that the early Inca Manco Cápac was instructed by the Sun to build Cusco “as a likeness and reflection of the house of the Sun, the Golden Garden.” In practice, planning was carried out by senior religious officials and master masons, while the physical work was performed by drafted communities of laborers, including stonemasons, lime burners, stone haulers, carpenters, and thatchers.

Standardization was a hallmark of imperial building. Walls were built to specified heights, with windows, niches, and doorways cut to canonical proportions. Doorways in imperial buildings are typically trapezoidal, with the upper lintel narrower than the lower threshold. Niches are recessed into walls in a uniform manner. Roofs were pitched and thatched, although in the most important temples they were capped with gold plates in pre-conquest times, as at the Coricancha. Public spaces were organized around rectangular plazas, while residential blocks followed standardized unit dimensions.

Building techniques

Inca builders developed a sequential, well-documented set of techniques for quarrying, transporting, shaping, and assembling stone. Although no Inca treatise on construction survives, the techniques can be reconstructed from archaeological evidence, the accounts of colonial chroniclers such as Bernabé Cobo, Juan de Betanzos, and Martín de Murúa, and from ethnohistorical studies of preserved and rebuilt walls in Cusco and elsewhere.

Stone was generally quarried within a few kilometers of the construction site, although at some sites, particularly at Ollantaytambo and at Sacsayhuamán, the stone was brought from specific quarries up to several kilometers away. The use of local stone had two practical advantages: it reduced the labor of transport, and it made finished buildings visually consistent with the surrounding bedrock. Major quarries associated with imperial projects include the quarries of Raqchi, from which blocks for several state buildings were extracted, the bedrock quarries above Ollantaytambo from which the six monoliths of the Temple of the Sun were taken, and the pinkish granite of the Quorior quarry near Chinchero.

Stones were detached from the parent rock through a combination of natural jointing, the careful use of fire, and the insertion of wooden wedges that were soaked with water. The expansion of the wood as it absorbed water split the rock along controlled lines. This technique, often described as “fire and water” quarrying, has been verified experimentally and observed in several Andean quarries. Once a block was detached, it was reduced in stages. A knapper first removed the rough excess with harder stone hammers, and the stone was then dressed with abrasives, including sand, water, and possibly volcanic ash, until its final form was achieved. In the imperial style, this final form was either a regular rectangular ashlar or, in the polygonal mode, a stone with multiple concave and convex faces.

The transport of dressed stones presented considerable logistical challenges. The largest megalithic blocks, such as the three stones of the Rumihuasi at Sacsayhuamán, each estimated to weigh between 120 and 200 tons, were moved several kilometers from their quarries to the construction site. Colonial chroniclers recorded that thousands of laborers cooperated to drag such blocks on log rollers. More commonly, however, individual stones weighed between several hundred kilograms and a few tons, well within the range that could be moved by teams of 12 to 40 men using ropes, levers, and wooden skids.

Once a stone reached the construction site, it was raised into position by a combination of ramps, levers, and cranes. Recent reconstructions of the Sun Temple at Ollantaytambo have shown that the joint between vertical members of a wall was typically cut and partially dressed at ground level, with the upper stones fitted into place during lifting. This required accurate surveying so that each stone met the dimensions of its neighbors to within a few millimeters.

Walls were not laid in continuous courses but followed what engineers call “scattered bond” construction: stones of varying sizes and shapes were placed so that the joints between any two courses did not align vertically. This technique, combined with the polygonal shaping of the stones themselves, distributed loads through the wall and produced an extremely rigid structure. Walls of the imperial type were typically built in three layers: an outer facing of carefully cut polygonal stones, a rubble and mud core, and a second facing on the interior. The core absorbed minor irregularities, and the outer layers carried the architectural appearance and structural load.

Doorways were typically trapezoidal, capped with a single lintel, and sometimes topped with an additional flat or trapezoidal stone for ornament. Windows were typically rectangular, with the same proportions as niches, and were often paired.

Stonework and ashlar masonry

Imperial Inca stonework is famous for the extreme precision of the joints between adjacent stones, which are so close that a knife blade or even a credit card cannot be inserted into them. The most refined example in Cusco is the Twelve-Angled Stone, a single block of green diorite fitted into a wall of the Hatunrumiyoc palace on Calle Hatunrumiyoc. The stone has twelve interior angles and thirteen exterior sides, each fitted precisely against its neighbors. It is a popular landmark and emblem of Cusco, although the wall in which it sits contains many stones of comparable precision, including examples with nine, ten, and eleven sides.

Ashlar masonry in the imperial style can be divided into several subtypes. Coursed ashlar uses rectangular stones laid in horizontal courses of varying height, found in some Inca coastal buildings and in works associated with the Chimú tradition. Polygonal masonry uses stones with multiple, irregular sides that interlock tightly with their neighbors. This is the characteristic style of imperial Cusco, of the Coricancha, and of most of the buildings of Machu Picchu. Cyclopean masonry describes walls built of very large, partially shaped blocks, used especially in the lower courses of Sacsayhuamán. Mosaic-style masonry is rare in the imperial tradition and is found mostly in Chachapoya architecture.

The cutting of polygonal stones, known in Quechua as pircado, was performed in three principal stages. First, the block was reduced to a rough shape, and a series of grooves was cut at the locations of the future joints. The stone was then fitted into a temporary position in the wall and tapped with a small hammer until the contact points with neighboring stones were identified. These contact points were then carefully hammered down, producing small circular depressions, or sillares, separated by a network of fine lines. The final fit was achieved by abrading the contact points with sand and water until the stone settled fully into place. This process could take several days for each individual stone and required a level of skill passed down within particular families and communities of stonemasons.

Modern studies, including the analysis of tool marks and the digital photogrammetry of famous walls, have shown that the cutting of polygonal masonry was a technically demanding craft, not a mystical feat as some earlier authors suggested. Tool marks indicate the use of harder stone hammers, copper and bronze tools, and abrasives. The precision of the joints has been the subject of numerous studies, including those by the Italian architect Enrico Cancian and the North American researchers Jean-Pierre Protzen and his students at the University of California, Berkeley, who carefully documented the techniques used at Ollantaytambo and at the Coricancha.

Mortarless construction and seismic resilience

Inca polygonal walls are built without mortar of any kind, with stones held together by their own weight, by gravity, and by the precise matching of their surfaces. This mortarless construction has a striking visual effect: the joints are sometimes so close that they appear to be cracks in a single piece of stone. The technique is, however, more than an aesthetic statement. By avoiding mortar, the Inca created walls that are capable of significant deformation under stress without losing integrity.

This characteristic became especially clear in 1950, when an earthquake of magnitude approximately 6.0 on the Richter scale struck the Cusco region on 21 May 1950. The earthquake destroyed many colonial buildings, including a significant portion of the cathedral, but Inca polygonal walls, including those of the Coricancha and the Sacsayhuamán site, generally withstood the shaking with little damage. Colonial-era observers had already noted this phenomenon, but the 1950 event prompted modern research.

Engineers, including those associated with the Pontificia Universidad Católica del Perú and later the Getty Conservation Institute, have shown that Inca walls absorb seismic energy by rocking on their foundations and by allowing adjacent stones to slide and then resettle into their original positions. The polygonal geometry, the inward batter of the wall face, and the presence of a rubble core that does not fully fill the gaps between facing stones all contribute to this behavior. The key feature is that the joints are not fixed but are designed to permit a small amount of motion.

The contrast between Inca and colonial buildings during the 1950 earthquake is striking in many places. At the church of Santo Domingo, which was built directly atop the Coricancha following the Spanish conquest, the colonial walls cracked and partially collapsed, while the Inca foundations remained intact. At the church of San Cristóbal in Cusco, built directly over an Inca foundation, the colonial structure failed in 1950, but the underlying Inca walls remained structurally sound. This pattern has been repeated in subsequent earthquakes, including the 1986 Cusco event and the 2007 Pisco earthquake, in which adobe and colonial masonry buildings suffered extensive damage while Inca constructions, where they were still standing, generally performed well.

This characteristic has led to a renewed interest in Inca building methods in the twenty-first century. Civil engineers working in seismic zones, including Japan, Italy, and Peru, have studied Inca joints in the laboratory. Some modern projects have drawn on the principles of Inca construction in the design of dry-stone retaining walls and low-rise buildings. Although polygonal ashlar is not practical in contemporary mass construction, the underlying principle of permitting controlled motion in stonework has influenced the design of rocking foundations and base-isolated structures.

The architecture of Cusco

Cusco, the capital of the Inca Empire, is the most concentrated repository of imperial architecture. The city was, in the late fifteenth century, one of the largest cities in the Americas, with an estimated population of perhaps 100,000 to 150,000 inhabitants at its peak. The imperial core of the city, the Huanaypata and Haucaypata plazas, is now the Plaza de Armas and the surrounding colonial blocks. Inca walls, foundations, and other remains are preserved in the colonial architecture above ground and have been investigated in numerous archaeological excavations.

The city underwent a major transformation under the ninth Sapa Inca, Pachacuti Inca Yupanqui. Earlier rulers had built modest structures, including the early Sun Temple, on the site of the later Coricancha. Pachacuti reorganized the city into the form of a puma lying on its side, with the head at Sacsayhuamán, the center at the Coricancha, and the tail at the confluence of the rivers. The body of the puma consisted of the residential and administrative compounds of the four suyus of the empire, while the lines of sight from one ceremonial precinct to another followed the spine of the animal. This urban design is described in the colonial chronicles of Garcilaso de la Vega, Pedro Cieza de León, and Bernabé Cobo, and it has been verified in part by modern archaeological surveys and a study of huacas within the city.

The plan of Cusco was oriented approximately 30 degrees east of true north, so that its principal axis pointed to the rising sun at the June solstice. The city’s principal building axes, including the Haucaypata and the Cusipata plazas, were aligned with this same direction. Major temples, fountains, and shrines were placed along these axes. The Sun Temple at the Coricancha, the Muru-kunka waterfall, and the Pumacurco aqueduct are among the ceremonial features arranged along the imperial geometry.

Sacsayhuamán

The fortress and ceremonial complex of Sacsayhuamán dominates the northern edge of Cusco. It occupies a hilltop ridge that rises roughly 200 meters above the city and was the ceremonial and defensive centerpiece of the imperial capital. The chroniclers agree that the complex was begun under Pachacuti Inca Yupanqui in the mid-fifteenth century, traditionally dated c. 1450, and that it took several decades to complete. Garcilaso de la Vega reports that 20,000 workers participated in its construction, and that the building of the fortress was the principal public works project of the late imperial period.

Sacsayhuamán consists of three sequential terraced walls that follow the natural contours of the hill. The lower walls are made of enormous cyclopean blocks; the upper walls are built of smaller, polygonal stones. The largest stones, on the upper courses, are the famous “three stones” of the Rumihuasi, each estimated to weigh between 120 and 200 tons, that fit together with such precision that a knife cannot be inserted between them. Recent archaeological and architectural work by the Universidad Nacional de San Antonio Abad del Cusco and by international projects has clarified the construction sequence: the lower cyclopean course was laid first, perhaps using stones dragged from quarries several kilometers away, and the upper polygonal courses were added later, perhaps as late as the reign of Huayna Capac.

The complex includes the great plaza of Muyu Mork’or or Muyu Murco, a large open space used for ceremonial dances and reviews of troops, and the so-called “throne” of the Inca, a carved stone seat. The complex was partially dismantled in the years following the Spanish conquest, when many of the smaller stones were used to construct colonial buildings in the city below and the Spanish demolished the upper towers in the 1530s and 1540s.

The Coricancha

The Coricancha, or “Golden Enclosure,” was the principal religious precinct of the Inca state. It was dedicated primarily to Inti, the sun god, and to Viracocha, the creator, but it also housed shrines to the moon (Mama Killa), the stars (Pleiades), thunder (Illapa), and the rainbow (Kuychi). The complex included a great central courtyard, temples dedicated to each of these deities, a garden of golden plants, a fountain, and a room dedicated to the mallquis, the mummified bodies of deceased rulers.

The Coricancha is known from archaeological remains, from the descriptions of colonial chroniclers, and from the site of the colonial church of Santo Domingo, which was built directly on top of the Inca precinct. The earliest versions of the precinct predate the empire; excavations in the early twentieth century by the Peruvian archaeologist Luis Valcárcel uncovered earlier walls beneath the polygonal masonry attributed to the Inca. Under Pachacuti, the complex was rebuilt in polygonal ashlar style, and the buildings were encased in gold plates, both flat and embossed, that were stripped by the Spanish in 1533.

The great wall that separates the precinct from the surrounding city is one of the most famous Inca walls in existence. It contains many of the most finely cut stones in the imperial corpus, including the famous stones of the “Hall of the Serpents” and the curved polygonal walls that mark the corner of the Sun Temple. The curved walls of the Sun Temple are particularly notable, as the polygonal technique is technically most challenging in a curved surface.

The Twelve-Angled Stone and the Puma design

The Twelve-Angled Stone is a single block of green diorite that forms part of a wall of the former Hatunrumiyoc palace, now a colonial residence on Calle Hatunrumiyoc in the historic center of Cusco. The stone is a celebrated example of imperial polygonal masonry and an emblem of the city. Although the wall in which the stone is set contains numerous stones of comparable complexity, the Twelve-Angled Stone has become a popular landmark and a symbol of Cusco and of Inca stonework in general. The stone is estimated to weigh approximately two tons and is fitted into a wall of polygonal stones of various sizes, each joint meeting its neighbors within fractions of a millimeter.

The puma design of Cusco is the cosmologically significant urban plan attributed to Pachacuti Inca Yupanqui. The chroniclers report that Pachacuti reshaped the city to look like a puma lying on its side, with the head at the fortress of Sacsayhuamán, the body in the central city, and the tail at the confluence of the Huatanay and Tullumayo rivers. The fortress of Sacsayhuamán is associated with the head and jaw of the puma, the Pumacurco aqueduct with the spine, and the central Sun Temple at the Coricancha with the navel (the literal meaning of Qosqo in Quechua). The design reflected a cosmological understanding of the city’s role as a microcosm of the empire and as a center of the Andean world.

Machu Picchu

Machu Picchu is the most famous Inca site and the best-known example of imperial architecture. It is located on a saddle between the peaks of Machu Picchu and Huayna Picchu, in the cloud forest of the eastern Andes, at an elevation of approximately 2,430 meters. The site was built under the reign of Pachacuti Inca Yupanqui, probably in the period c. 1450, as a royal estate dedicated to the cult of the emperor, the cult of the sun, and possibly the cult of the Apus, the mountain spirits. It is estimated to have housed up to 750 permanent residents, including priests, aqllakuna (chosen women), craftsmen, and servants, and supported a much larger population in its agricultural hinterland.

Machu Picchu was unknown to the Spanish during the conquest and was not rediscovered by the outside world until Hiram Bingham, a North American historian, brought it to international attention in 1911. The site is laid out on a sequence of terraces cut into the mountain flank, with a clear separation between the urban (Hanan) and residential (Hurin) sectors. The site includes approximately 172 buildings, including temples, royal residences, storehouses, workshops, and a series of fountains that channel water from a spring above the city.

The principal religious buildings of the site include the Intihuatana stone, the Temple of the Sun (a curved polygonal structure), the Room of the Three Windows, the Principal Temple, and the Unfinished Temple. The Intihuatana is a carved granite outcrop that is believed to have served as an astronomic observation point, marking the solstices and the equinoxes. The Temple of the Sun, built atop a large natural boulder, is a semi-circular structure built with the same polygonal ashlar technique used at the Coricancha. The Room of the Three Windows is a long hall with three trapezoidal openings aligned to provide views of the surrounding mountains.

The engineering of Machu Picchu is striking for its integration of natural and built form. The site sits on a fault zone, and Inca builders made use of a deep fracture in the bedrock to channel drainage and to provide a foundation for the Sun Temple. The site is crossed by a sequence of stepped agricultural terraces that helped stabilize the slopes against erosion and landslides, and these terraces were also used to grow maize on the lower levels and potatoes on the upper levels. A complex system of stone-lined drainage channels and agricultural terraces, described in detail in modern studies, prevented the site from being damaged by heavy rainfall.

The site was abandoned in the early colonial period, probably after 1532. Bingham’s team cleared and reconstructed part of the site during their 1912 excavations. Peru has protected Machu Picchu as a national archaeological park since 1961, and the site was designated a UNESCO World Heritage Site in 1983.

Ollantaytambo

Ollantaytambo is a town and archaeological site in the Sacred Valley of the Urubamba, located approximately 60 kilometers northwest of Cusco. The site is a complex of ceremonial and defensive buildings built on a steep ridge and a series of agricultural terraces on the valley floor. The town is one of the few Inca settlements that have been continuously inhabited since the imperial period; the lower town retains its original Inca street plan and is recognized as a National Historic Landmark.

The site has a complex building history. The lower terraces and the town of Ollantaytambo are attributed to the ninth Sapa Inca, Pachacuti Inca Yupanqui, who used the site as a royal estate. The principal buildings on the upper terrace include the Temple of the Sun, a series of structures built with large polygonal blocks, and the unfinished ceremonial buildings on the ñusta hill. The six monoliths of the Sun Temple, each weighing approximately 50 tons, were quarried from the mountainside above the site and hauled several kilometers to their current position. The transport of these stones, and the construction of the upper terraces, was abandoned before the complex was completed, possibly because of the civil war of 1529–1532 or the Spanish conquest of 1532.

Ollantaytambo is famous as the site of a battle fought by the Inca general Manco Inca in 1537. Manco Inca, who had been crowned at Cusco by the Spanish in 1533 but had rebelled in 1535, occupied the fortress and routed a Spanish force under Juan Pizarro in 1537. The victory was not decisive, and Manco Inca ultimately withdrew to Vilcabamba, but the engagement demonstrated the continued military effectiveness of the Inca army and the strength of the Ollantaytambo fortifications. The site was never taken by Spanish assault, and it was abandoned by Inca forces only after Manco Inca’s withdrawal.

The engineering of Ollantaytambo is notable for its sophisticated use of the steep topography. The site includes a series of stone-lined canals and channels that direct water from the Patakancha River to the lower terraces and to a series of fountains in the town. The terraces are built of polygonal andesite blocks that have weathered more than 500 years of agricultural use. The town plan, which is preserved in the lower town, includes long, narrow residential blocks organized around internal courtyards. The orientation of the buildings follows astronomical alignments, including the rising of the Pleiades.

Choquequirao

Choquequirao is a large Inca site in the Vilcabamba region, located on a ridge above the Apurímac River at an elevation of approximately 3,050 meters. The site was the third-largest Inca complex in the Cusco region, after Cusco and Machu Picchu, and is built on a much larger footprint than the more famous site, although it is much less well known. It is reached by a difficult two-day trek from the town of Cachora, although access is improving with a planned cable car and a road.

The site is conventionally dated to the mid-fifteenth century and is attributed to the reign of Túpac Inca Yupanqui, the tenth Sapa Inca. It is interpreted as a royal estate, similar in function to Machu Picchu, and as a religious center dedicated to the cult of the Apus and of the Pachamama, the earth mother. The complex includes more than 180 buildings, including temples, royal residences, agricultural terraces, fountains, and a series of geometric groups of structures that are believed to represent llamas in the form of the chaquip.

Choquequirao is divided into two main sectors. The upper sector, on the western ridge, includes the principal religious and ceremonial buildings, while the lower sector, on the eastern ridge, includes agricultural terraces and buildings of the llacta. The site is also notable for its sophisticated water management: a sequence of stone-lined canals and channels carries water from a natural spring to a series of fountains, including a long channel known as the Amaicha that runs nearly the full length of the site. The site is currently being studied and partially excavated by the Peruvian Ministry of Culture and by international projects.

Hydraulic engineering

Inca hydraulic engineering included the design and construction of aqueducts, irrigation canals, drainage systems, fountains, and water-powered devices. The Inca state invested heavily in the rational distribution of water, both for ceremonial purposes and for agriculture. Water was considered a sacred substance, and the camayoc in charge of water infrastructure held positions of high status. Water sources were protected as huacas, and offerings were made to springs, rivers, and lakes at the beginning and end of the agricultural season.

The Cusco water system included a sophisticated network of canals and fountains that brought water from surrounding springs into the city. The principal canals include the Pumacurco aqueduct, which carried water from a spring on the hillside of Pumacurco to the Coricancha, and the Antarumi canal, which delivered water to the markets and to a fountain in the Huanaypata plaza. The Sacred Valley irrigation system included major canals such as the Pampamarca canal and the Chinchero canal, which delivered water to the agricultural terraces of the lower Sacred Valley. These canals, often cut into bedrock or lined with polygonal stones, are still in use today in some areas, although they have been modified and extended by Spanish and modern interventions.

Tipón, located in the Lucre basin southeast of Cusco, is the best-preserved example of Inca hydraulic engineering. The site includes a series of terraces, ceremonial buildings, and a complex of channels and fountains that bring water from an underground spring to a sequence of pools and outlets. The hydraulic system of Tipón was studied in detail by the American engineer Kenneth Wright and his team, who demonstrated that it includes an upstream drainage system, a main canal, drop structures, and a sequence of waterfalls, fountains, and pools. The system was designed to control erosion, distribute water, and produce a visually striking display of the camayuc, the life-giving water.

The drainage systems of imperial sites such as Machu Picchu include a sequence of stone-lined channels, drainage grates, and the use of the natural topography to direct water away from buildings. At Machu Picchu, 129 separate drainage channels handle the heavy rainfall of the cloud forest, and the lower courses of the walls are set on bedrock and on stone-lined foundations that resist the impact of water.

Inca hydraulic engineering also included a sophisticated use of andenes, or agricultural terraces, which are described in the next section. Terraces slowed the flow of water, retained moisture, and reduced soil erosion, and the Inca understanding of water was integrated with the broader Andean cosmology in which water, soil, plants, and humans were all part of a single system of mutual dependencies.

Agricultural terraces (andenes)

The Inca built more than 1 million hectares of agricultural terraces across the empire, particularly in the highlands of Peru, Bolivia, and Ecuador. Terraces transformed steep, often arid slopes into arable land by creating flat platforms backed by retaining walls and filled with layers of stones, gravel, and topsoil. The Inca developed several distinct types of terrace: simple field terraces for level ground, sloped or inclined terraces on gentle slopes, and steeply walled or pisqa terraces on near-vertical slopes. Each was used in a different environmental context.

The internal structure of Inca terraces was based on a series of layers. The retaining wall, which was typically built of polygonal stone, was followed by a layer of large stones (the piedra brava), which provided drainage, a layer of q’illpa stones, which retained moisture, a layer of sandy soil, and finally a layer of topsoil, which was enriched with the deliberate incorporation of gravel and organic matter. The piedra brava layer, which is the largest of the three subsoil layers, can be more than 2 meters thick in large terraces. The q’illpa layer, often composed of volcanic or glacial deposits, retains moisture and prevents the topsoil from washing down to the drainage layer. Modern archaeological and soil studies, including work at Moray and at Pisaq, have shown that this layered structure could support intensive cultivation for many decades.

Terraces in the Sacred Valley of the Urubamba are particularly well preserved. The Pisac terraces, built on a steep mountainside above the town of Pisac, are among the most extensive in the region and include a sequence of more than 30 levels. The terraces are still in use and produce maize, potatoes, quinoa, and a range of other crops. The Chinchero terraces, on the plateau above the town of Chinchero, are also in use and are part of an integrated system that includes irrigation canals, the so-called labyrinth terrace, and a series of ritual structures.

Moray, a few kilometers from the town of Maras, is a unique group of four circular depressions, each of which contains concentric circular terraces. The depressions are natural, but the Inca built the circular terraces to a depth of more than 30 meters in the largest case. The temperature difference between the top and bottom of the largest depression can be as much as 4–5°C, and the various terraces create a range of microclimates. It is interpreted as an agricultural laboratory, in which the Inca experimented with the cultivation of different crops at different altitudes and in different microclimates, and the site is one of the most photographed Inca monuments in the region.

The terracing system of the Inca has drawn modern attention for its resilience. The terraces were used continuously from the time of the Inca until the twentieth century, and in many cases they are still in use. The agricultural terraces of the Colca Valley in southern Peru, the Inca terracing of the Cotahuasi Canyon, and the terracing of the Sacred Valley are recognized for their integration of architecture, hydrology, and soil management, and they have inspired contemporary efforts to revive traditional agricultural techniques in the Andes.

Storage and the qollqa system

The Inca developed a sophisticated system of state storage to provide food, raw materials, and ritual goods for the population. The system, which is known through archaeology and through the accounts of colonial chroniclers, was organized by the central state and included thousands of storehouses, or qollqas, distributed across the empire. The qollqas were typically built of stone, with a half-buried floor, a single narrow doorway, and a stone or thatched roof. They were designed to be cool, dry, and well-ventilated, and they used the cool temperatures of high altitudes to preserve maize, quinoa, dried potatoes, and other staples.

The qollqas of the Tipón archaeological site, in the Lucre basin southeast of Cusco, include more than 280 individual storehouses, organized into groups on a series of terraces. The Tipón site is one of the most extensive and best-preserved examples of Inca state storage. The individual storehouses are arranged in a grid, and the complex is laid out in a way that allows for a steady flow of goods in and out. The site is currently being studied by the Peruvian Ministry of Culture and by international projects.

The Inca stored maize, quinoa, dried potatoes, dried llama meat, coca leaves, cotton, and other goods. The state took a share of agricultural production as a tax, stored it in the qollqas, and distributed it to the population in years of poor harvest, as a form of insurance, and to soldiers and laborers on imperial projects. The colonial chronicler Pedro Cieza de León recorded that the Inca were able to feed populations for years from these state stores. The storage system also supported the imperial army, the mit’a labor system, and the aqllakuna, the chosen women, who lived in cloisters in the major cities.

The state granaries of the Inca were used to support the imperial cult, the imperial army, and the imperial administration. They were placed near important agricultural areas, near major roads, and at the frontiers of the empire. The largest of the imperial granaries, the qollqas of Colcamayo, near the town of Juliaca, included more than 600 individual storehouses and could hold approximately 5,000 tons of grain. The granary system, in this respect, was a major reason for the resilience of the Inca economy.

The Spanish made use of the Inca qollqas after the conquest, but they did not maintain the system in the same way. Over the centuries, many of the qollqas have been destroyed, modified, or left to decay. However, several thousand individual storehouses are still in evidence across the Andean region, and the network of storage sites is one of the most extensive archaeological records of the Inca state.

The Sacred Valley

The Sacred Valley of the Urubamba, sometimes called the “Sacred Valley of the Incas,” is a stretch of the upper Urubamba River valley northwest of Cusco, between the towns of Pisac and Ollantaytambo. The valley was the agricultural heartland of the Inca state and contained some of the most important imperial estates. It is a region of mild climate, fertile soil, and abundant water, and it was the site of large imperial estates that supplied the city of Cusco and the royal panaca, or royal lineage cult.

Pisac is a town and archaeological site in the upper Sacred Valley. The site includes a fortress on a mountain ridge, a sequence of agricultural terraces, a cemetery of the Inca elite, and the foundations of imperial buildings. The fortress, which includes ceremonial and defensive structures, is built on a series of platforms and overlooks the valley. The terraces, on the slopes below the fortress, are among the most extensive in the region. The cemetery, on the opposite side of the valley, contains hundreds of burials in deep crevices and caves.

Chinchero, on the plateau above the valley, is the site of a large Inca town, a colonial church built on Inca foundations, and an extensive system of agricultural terraces. The colonial church, dedicated to the Nativity of Our Lady, was built on the foundations of an Inca building and contains some of the largest known Inca polygonal stones in the region.

The town of Urubamba, the largest settlement of the Sacred Valley, is a modern town, but it lies at the center of the agricultural land of the Inca. The valley is now a major tourist destination and one of the most visited regions of Peru.