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.