Just a short, steep climb from the heart of Cusco, you'll find the incredible fortress of Saqsaywaman watching over the city. The sheer scale of the massive, interlocking stone walls is something you have to see to believe—how the Inca fit these giant boulders together so perfectly without mortar is a true mystery. Take your time wandering through the ruins and be sure to climb to the top for some of the most spectacular panoramic views of the entire valley. The walk up from the Plaza de Armas takes about 30-45 minutes and is a bit of a workout due to the altitude, but it's absolutely worth it.