During the Inca Empire, Ollantaytambo was the royal estate of Emperor Pachacuti who conquered the region, built the town and a ceremonial center around 1440 CE. The “fortress” of Ollantaytambo is actually a religious structure, but it did function as a retreat from the attacks of the conquistadores in the 1500’s. It was last held by Manco Inca, who, leading resistance forces against the Spanish, retreated to Ollantaytambo in January of 1537. The Spanish forces attacked on horseback, but Manco Inca’s band, in a technical tour de force, flooded the entire approach plain with water forcing the Spanish to retreat and regroup. Though Ollantaytambo was a highly effective fortress, it also served as a temple. A finely worked ceremonial center is at the top of the terracing. Some extremely well-built walls were under construction at the time of the conquest and have never been completed.
Machu Picchu was built around 1450, at the height of the Inca Empire. Its construction appears to date to the period of the two great Incas, Pachacutec Inca Yupanqui (1438–71) and Tupac Inca Yupanqui (1472–93). It was abandoned just over 100 years later, in 1572, as a belated result of the Spanish Conquest. Machu Picchu is situated above a bow of the Urubamba River, which surrounds the site on three sides, with cliffs dropping vertically for 450 meters (1,480 ft) to the river at their base. The area is subject to morning mists rising from the river. The location of the city was a military secret, and its deep precipices and steep mountains provided excellent natural defenses. The Inca Bridge, an Inca grass rope bridge, across the Urubamba River in the Pongo de Mainique, provided a secret entrance for the Inca army. The city sits in a saddle between the two mountains Machu Picchu and Huayna Picchu, with a commanding view down two valleys and a nearly impassable mountain at its back. It has a water supply from springs that cannot be blocked easily, and enough land to grow food for about four times as many people as ever lived there. To say Machu Picchu was an impregnable fortress in addition to a sanctuary is to overstate the obvious.