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.
Machu Picchu was built in the middle of the 15th century at the command of Pachacutec, ninth ruler of the Tahuantinsuyo and the one responsible for expanding it into what we call the Incan Empire. He was just twenty years old when he ascended the throne of his father, Viracocha, after he led the Incan armies in their defeat of the warlike Chancas. Wearing a colorful headdress, Pachacutec began his long reign that transformed the Andean world and built Cusco into the amazing city that left the Spanish Conquistadores stupefied. Seen in isolation from Europe or North America, it seems to be a marvelous anomaly, a city built on top of a mountain in a remote cloud forest, for unknown and possibly mysterious reasons. To accomplish this amazing feat, the builders had to remove massive quantities of stone and dirt and to construct enormous terraces, canals, foundations that were several meters deep, and monolithic walls of remarkable polished stones. It is believed that the city may have been used as a hiding place when the Spanish invaded, and despite the efforts made to locate it, Machu Picchu was never discovered by the Spaniards. Still, it is believed that the reason the city fell was due to the smallpox brought over by the Spanish, and that in trade with Cusco, the disease was transmitted. Less than a hundred years after being built it was abandoned.