The Hagia Sophia is a former Orthodox patriarchal basilica, later a mosque and now a museum. Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and to have "changed the history of architecture." It was the largest cathedral in the world for nearly a thousand years, until the completion of the Seville Cathedral in 1520. The current building was originally constructed as a church between 532 and 537 A.D. on the orders of the Byzantine Emperor Justinian.