Hagia Sofia is the former Greek Orthodox Christian patriarchal cathedral, later an Ottoman imperial mosque and now a museum (Ayasofya Müzesi) in Istanbul. Built in AD 537 at the beginning of the Middle Ages, it was famous in particular for its massive 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”
In 1453, Constantinople was conquered by the Ottoman Empire under Mehmed the Conqueror, who ordered this main church of Orthodox Christianity converted into a mosque. Although some parts of the city of Constantinople had fallen into disrepair, the cathedral had been maintained with funds set aside for this purpose, and the Christian cathedral made a strong impression on the new Ottoman rulers who conceived its conversion. The bells, altar, iconostasis, and other relics were destroyed and the mosaics depicting Jesus, his Mother Mary, Christian saints, and angels were also destroyed or plastered over. Islamic features – such as the mihrab (a niche in the wall indicating the direction toward Mecca, for prayer), minbar (pulpit), and four minarets – were added. It remained a mosque until 1931 when it was closed to the public for four years. It was re-opened in 1935 as a museum by the Republic of Turkey. Hagia Sophia was, as of 2014, the second-most visited museum in Turkey, attracting almost 3.3 million visitors annually. According to data released by the Turkish Culture and Tourism Ministry, Hagia Sophia was Turkey’s most visited tourist attraction in 2015.
From its initial conversion until the construction of the nearby Sultan Ahmed Mosque, aka the Blue Mosque of Istanbul, in 1616, it was the principal mosque of Istanbul. The Byzantine architecture of the Hagia Sophia served as inspiration for many other Ottoman mosques, including the Blue Mosque, the Şehzade Mosque, the Süleymaniye Mosque, the Rüstem Pasha Mosque and the Kılıç Ali Pasha Complex.