Machu Picchu. 2,400mtrs or 7,875feet
Viewed from above it becomes clear that, with careful water management, the Incas used every square inch of ground, and it looks like you could live there. I am told it could provide food for four times the estimated population. Seen from below it looks more like the "dressed" mountain it is. The Incas managed to turn a near vertical slope into a town with fields for crops and gardens. Perhaps the wheel was not used, except as a toy, because they would be unusable on the slopes, and with the few flat bits the Inca leg muscles wouldn't have needed assistance. If any wheelbarrows were ever used here, their remains would inevitably be at the bottom of the hill, waiting for an imaginative archaeologist. The Religious and Regal structures with their mortar-free building technique are in excellent condition, the damage on the right here is caused by a tree, (later removed by Bingham's crew,) prizing the stones apart, rather than by earthquake. The buildings for the lesser folk used mortar and sustained some damage, but their main salvation has been not being turned into a cathedral.
Wikipedia has some good stuff here.
Apparently MP gets fairly busy in the high season, but it was acceptable when we were there and the overcast skies prevented sunburn while made the mountains more atmospheric. Many tourists arrive, visit, and leave on the same day, that means the place is much emptier early and late, and an overnight stay also gives midday access to the extensive market in Aguas Calientes when the site is overcrowded