As you look past Machu Picchu to Huayan Picchu, you see a mountain covered with plants. Yet at the top of the mountain there are many Inca ruins. No one seems to know exactly why the buildings are there. The most common believe is that the buildings were used as an outpost to survey the surroundings for possible enemy movements. But again, because the Incas had no written language, there exact purpose will never be known.
Looking at the people in these photos, I think you can get some idea as to how hard the climbing is.