4. Hagia Sophia, Istanbul
Hagia Sophia is a former Orthodox patriarchal basilica, later a mosque, and now a museum in Istanbul, Turkey. From the date of its dedication in 360 until 1453, it served as the Greek Patriarchal cathedral of Constantinople, except between 1204 and 1261, when it was converted to a Roman Catholic cathedral under the Latin Patriarch of Constantinople of the Western Crusader established Latin Empire. The building was a mosque from 29 May 1453 until 1931, when it was secularized. It was opened as a museum on 1 February 1935. Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and is said to have “changed the history of architecture.” It was the largest cathedral in the world for nearly a thousand years, until Seville Cathedral was completed in 1520. In 1453, Constantinople was conquered by the Ottoman Turks under Sultan Mehmed II, who subsequently ordered the building converted into a mosque.