Istanbul is historically important as the capital first of the Byzantine Empire and later of the Ottoman Empire. In its influence on the development of Roman law, Greek philosophy and art, and Christian theology and on church history, Byzantine-ruled Istanbul rivalled the cultural contributions of Athens, Jerusalem, Rome, and Paris.

Agia Sophia (Istanbul)
Agia Sophia (Church of the Holy Wisdom) was built in Constantinople (now Istanbul) between 532 and 537 under the auspices of Emperor Justinian I. Innovative Byzantine technology allowed architects Anthemius of Tralles and Isidore of Miletus to design a basilica with an immense dome over an open, square space, pictured. The original dome fell after an earthquake and was replaced in 563. The church became a mosque after the Ottoman conquest of 1453, and is now a museum.

It was Justinian's great church at Constantinople, Agia Sophia, that demonstrated how to place a vast dome over a square plan. The solution was to place the dome on pendentives, or spherical triangles, that make a circle out of the square by rounding its corners.

Byzantine churches, each with a central dome opening into surrounding semidomes and other vault forms, and accompanied by the characteristic iconography, proliferated throughout the Byzantine Empire—Greece, the Balkans, Asia Minor, and parts of North Africa and Italy—and also influenced the design of churches in Western Christendom. Later churches are often miniaturizations of the original grandiose concept.