When he Roman Empire fell in 476 AD the Middle Age started. However, before its fall, the empire was divided into the Eastern and Western Roman Empires. In 476, the “Western Roman Empire” fell, while the Eastern Roman Empire, whose capital became Constantinople, preserved Roman culture (and architecture) and became the Byzantine Empire.
The religious buildings and their designs are the first achievements of the Byzantine Empire. Another important advancement was the development of bacilicas. Bacilicas were early Christian or medieval churches. This style was common in Roman Catholic and Eastern Orthodox churches.
Byzantine and Islamic architecture share a common trend: that is, the use of the dome. One example is the Dome of the Rock in Jerusalem, which was Islamic architecture, but illustrates the influence Byzantine bestowed as the dome style passed on to the Muslims. They often used “Persian’ dome. We modernly refer to this as the Onion Dome. The most celebrated example is the Taj Mahal (A.D. 1630) at Agra, India. Byzantine’s advancement in developing the dome created a new style in global architecture, for no other civilization had designed buildings, especially religious buildings, as did the Byzantines.
As early as the building of Constantine’s churches in Palestine there were two chief types of plan in use: the basilican, or axial, type, represented by the basilica at the Holy Sepulchre, and the circular, or central, type, represented by the great octagonal church once at Antioch. Those of the latter type we must suppose were nearly always vaulted, for a central dome would seem to furnish their very raison d’etre. The central space was sometimes surrounded by a very thick wall, in which deep recesses, to the interior, were formed, as at the noble church of St George, Salonica (5th century), or by a vaulted aisle, as at Sta Costanza, Rome (4th century); or annexes were thrown out from the central space in such a way as to form a cross, in which these additions helped to counterpoise the central vault, as at the Mausoleum of Galla Placidia, Ravenna (5th century). The most famous church of this type was that of the Holy Apostles, Constantinople. Vaults appear to have been early applied to the basilican type of plan; for instance, at Hagia Irene, Constantinople (6th century), the long body of the church is covered by two domes.
The Golden Age of Byzantine Architecture was under the rule of Justian in 527-565. It was during this period that the most famous examples of all Byzantine Architecture was built, including the Hagia Sophia. The Hagia Sophia was rebuilt from 532 to 537 after the previous church was destroyed by riots and fires. This church has some unique features which became the patterns for Byzantine Architecture for years after.
The style of the Hagia Sophia or Church of Divine Wisdom, was to have a large dome in the middle of the structure. The dome has a unique form in that it rest on 4 massive pillars which are arranged in a square.