Inca builders dressed stone into ashlar blocks ground to fit so precisely that mortarless walls held by gravity and matched faces alone, famously without a knife-blade gap. Several features made the work earthquake-resistant: trapezoidal doors and niches wider at the base, walls battered slightly inward, rounded corners and interlocking L-shaped blocks that keyed adjacent walls together. At Sacsayhuaman above Cusco some blocks weigh around 200 tonnes. Many Inca walls have outlasted the colonial masonry later raised upon them through repeated seismic events.
It is a distinct, empirically seismic structural tradition that shaped the monumental fabric of Cusco, Ollantaytambo and Machu Picchu. The trapezoidal aperture became a signature of Andean state architecture.