<P> Architecturally, Inti Mach'ay is the most significant structure at Machu Picchu . Its entrances, walls, steps and windows are some of the finest masonry in the Incan Empire . The cave also includes a tunnel - like window unique among Incan structures, which was constructed to only allow sunlight into the cave during several days around the December solstice . For this reason, the cave was inaccessible for much of the year . Inti Mach'ay is located on the eastern side of Machu Picchu, just north of the "Condor Stone ." Many of the caves surrounding this area were prehistorically used as tombs, yet there is no evidence that Mach'ay was a burial ground . </P> <P> The central buildings use the classical Inca architectural style of polished dry - stone walls of regular shape . The Incas were masters of this technique, called ashlar, in which blocks of stone are cut to fit together tightly without mortar . </P> <P> The section of the mountain where Machu Picchu was built provided various challenges that the Incas solved with local materials . One issue was the seismic activity due to two fault lines . It made mortar and similar building methods nearly useless . Instead, the Inca mined stones from the quarry at the site, lined them up and shaped them to fit together perfectly, stabilizing the structures . Inca walls have many stabilizing features: doors and windows are trapezoidal, narrowing from bottom to top; corners usually are rounded; inside corners often incline slightly into the rooms; and outside corners were often tied together by "L" - shaped blocks; walls are offset slightly from row to row rather than rising straight from bottom to top . </P> <P> Heavy rainfall required terraces and stone chips to drain rain water and prevent mud slides, landslides, erosion and flooding . Terraces were layered with stone chips, sand, dirt and top soil, to absorb water and prevent it from running down the mountain . Similar layering protected the large city center from flooding . Multiple canals and reserves provide water throughout the city that could be supplied to the terraces for irrigation and to prevent erosion and flooding . </P>

Where did the machu picchu stones come from