The Hagia Sophia. Probably Istanbul's most famous landmark, the Hagia Sophia (also spelled Ayasofya) was built by the emperor Justinian I in the year 537 AD. Built in only six years, the structure was designed by the architects Anthemius of Tralles and Isidore of Miletus. On May 7, 558, the dome of the church collapsed due to a December 557 earthquake, and though a new dome was quickly rebuilt, historical records tell us that it was not identical to the original.