Look at the size of these boulders near the top. Each weigh 150-200 tons. The small rocks between them allow for shifting of the huge stones. These were put together with brass "keys", needed for stability. Only problem is, the Incas did not use keys to hold their blocks together, they didn't need to with their precision.
Soooo, these stones were not placed here by the Incas but some "tribe" who was here BEFORE the Incas. The Incas actually built on an existing site. On the small block lying down, you can see where the key went.