Hagia Sophia, a former Greek Orthodox Christian patriarchal basilica (church), later an Ottoman imperial mosque and now a museum (Ayasofya Muzesi) in Istanbul, Turkey. Built in 537 AD at the beginning of the Middle Ages, it was famous in particular for its massive dome. From the date of its construction in 537 AD until 1453, it served as an Eastern Orthodox cathedral and the seat of the Patriarch of Constantinople, except between 1204 and 1261, when it was converted by the Fourth Crusaders to a Roman Catholic cathedral under the Latin Empire. The building was later converted into an Ottoman mosque from 29 May 1453 until 1931