Historians believe Machu Picchu was built at the height of the Inca Empire, which dominated western South America in the 15th and 16th centuries. It was abandoned an estimated 100 years after its construction, probably around the time the Spanish began their conquest of the mighty pre-Columbian civilization in the 1530s. There is no evidence that the conquistadors ever attacked or even reached the mountaintop citadel, however; for this reason, some have suggested that the residents’ desertion occurred because of a smallpox epidemic.