Sacsayhuaman is an impressive Inca fortress on a steep hill that overlooks all of Cusco. The ruins are humongous, but archeologists believe that the original site was as much as four times larger. What remains today are the impressive outer walls constructed in a zigzag formation across three levels. As with many Inca sites, the walls are made from massive, irregularly-shaped boulders that stick together like a jigsaw puzzle without any additional support. The stones are laid together so tightly that a sheet of paper will not fit into many of the cracks. As the night comes down, this is a perfect location to appreciate the stars.