Machu Picchu was built in a classic Inca architectural style of polished dry-stone walls. Its primary buildings are the Intihuatana, the Temple of the Sun and the Room of the Three Windows located in what is known by archaeologist as the Sacred District of Machu Picchu. There are concerns about the impact of tourism to the site as its visitors reached 400,000 in 2003. On September 2007, Peru and Yale University reached an agreement regarding return of artifacts removed from Macchu Picchu in the early 20th century by Hiram Bingham.Machu Picchu was constructed around 1450 in the andean mountains of Peru, at the height of the Inca Empire, and was abandoned less than 100 years later, as the empire collapsed under Spanish conquest. One theory maintains that Machu Picchu was an Incan “llacta”: a settlement built up to control the economy of the conquered regions and that it may have been built with the purpose of protecting the most select of the Incan aristocracy in the event of an attack. Based on research conducted by scholars such as John Rowe and Richard Burger, most archaeologists now believe that, rather than a defensive retreat, Machu Picchu was an estate of the Inca emperor Pachacuti. Johan Reinhard presents evidence that the site was selected based on its position relative to sacred landscape features, especially mountains that are in alignment with key astronomical events.