Istanbul's Hagia Sophia (Greek) or Ayasofya (Turkish) was originally constructed as a main Eastern Orthodox church and served in this role from 537 CE until the fall of Constantinople in 1453 (except between 1204 and 1261 when it was converted by the Fourth Crusaders to a Roman Catholic cathedral).

When the Ottoman Turks under Mehmed II conquered Constantinople, the Hagia Sophia was converted into a mosque and Christian relics and art were either removed or plastered over. It remained a mosque for almost 500 years, before being converted into a museum between 1931 and 1935.

Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture and influenced the design of numerous mosques in what is now Istanbul.