Hagia Sophia, or Aya Sofya (Turkish), is the former Greek Orthodox Christian patriarchal cathedral, later an Ottoman imperial mosque and now a museum (Ayasofya Müzesi) in Istanbul, Turkey. Built in AD 537, during the reign of Justinian. It is famous for its large dome. It was the world's largest building and an engineering marvel of its time. It is considered the epitome of Byzantine architecture and is said to have "changed the history of architecture."