"The Byzantine Church of Hagia Sophia stands atop the first hill of Constantinople at the tip of the historic peninsula, surrounded by the waters of the Sea of Marmara, the Bosphorus and the Golden Horn on three sides. It was built by Justinian I between 532 and 537. . .

As the Cathedral of the Ecumenical Patriarchate of Constantinople for over a thousand years, with the brief exception of the Latin occupation, the Hagia Sophia was the center of Eastern Christianity from 360 to the Ottoman conversion. Its importance as the center of religious authority in the Byzantine capital was compounded with its role as the primary setting for state rituals and pageantry. The Ottoman conquest of Constantinople in 1453, which put an end to the Byzantine Empire, began the era of Islamic worship in the holy structure, which Mehmed II converted into a mosque immediately after his conquest.

Known then on as the Ayasofya Mosque, the Hagia Sophia remained the Great Mosque of the Ottoman capital until its secularization under the Turkish Republic In 1934."

From The History of Hagia Sophia on ArchNet.org. Full artical here: hagiasophia_archnet.html