The central buildings of Machu Picchu 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. The Incas were among the best stone masons the world has seen, and many junctions in the central city are so perfect that it is said not even a knife blade fits between the stones. Terraced Fields of Machu Picchu Some Inca buildings were constructed using mortar, but by Inca standards this was quick, shoddy construction, and was not used in the building of important structures. Peru is a highly seismic land, and mortar-free construction was more earthquake-resistant than using mortar. The stones of the dry-stone walls built by the Incas can move slightly and resettle without the walls collapsing. Inca walls show numerous design details that also help protect them from collapsing in an earthquake. Doors and windows are trapezoidal and tilt inward from bottom to top; corners usually are rounded; inside corners often incline slightly into the rooms; and "L"-shaped blocks often were used to tie outside corners of the structure together. These walls do not rise straight from bottom to top but are offset slightly from row to row [Wikipedia.org]