The other distinctive feature of Inca architecture is trapezoidal doors and niches. As John Hemming writes in The Conquest of Incas:
Doors and niches were invariably built in trapezoidal shapes, with the sides tapering inwards towards the lintel at the top. This was a logical method for builders who had not discovered the principle of the arch. It reduced the length of the lintel stone and spread the thrust of the weight it supported. Rows of such trapezoidal niches broke the monotony of Inca walls. Sometimes the niches were the size of sentry-boxes, tall enough to accommodate a line of standing attendants, but more often they were smaller, sunk into the wall at chest height to form a row of convenient cupboard alcoves.
Examples above of a trapezoidal door (from Saqsayhuaman) and wall alcoves (from Machu Picchu).