Istanbul is home to the Hagia Sophia, built as a Byzantine cathedral in the sixth century.
The present Hagia Sophia was commissioned by the Byzantine emperor Justinian I after the previous church on the site was destroyed during the Nika riots. It was completed and dedicated in 537, becoming the principal cathedral of Constantinople, the Byzantine capital then known as Constantinople.
Its enormous dome, pendentives, and complex structural design made it one of late antiquity’s greatest architectural achievements. The building served as an Eastern Orthodox cathedral for centuries, except during the Latin occupation of Constantinople from 1204 to 1261, when it was used as a Roman Catholic cathedral.
After the Ottoman conquest of Constantinople in 1453, Hagia Sophia became a mosque. In 1935, the Turkish Republic opened it as a museum. Turkey converted it back into a mosque in 2020, while keeping it open to visitors under current arrangements.
The city itself was officially renamed Istanbul in international usage after the Ottoman period; Constantinople remains the historic name associated with the Byzantine era.