The Byzantine, or Eastern Roman, Empire, began with the foundation of Constantinople (formerly Byzantium) in ad 324 and ended with its capture by the Ottoman Turks in 1453. The Byzantine style began in the age of Justinian (527–65), although elements can be found from C4, and continued long after the fall of Constantinople, especially where the Orthodox Church was dominant. When the Roman Emperor Constantine (324–37) established his new Imperial and administrative capital on the Bosphorus, the seeds were sown for a division of the Empire into Eastern and Western parts, with Greek becoming dominant in the former and Latin in the latter. The division was exacerbated in C11 when Christendom suffered its Great Schism, dividing into the Orthodox and RC Churches (the latter centred on Rome).
When Constantinople was founded, every effort was made to create a new Rome in the East. Many Roman buildings were plundered to enrich the city, and the Classical Orders were familiar there, as well as the style of architecture which we call Early Christian. However, two building-types played an important part in the evolution of a specifically Byzantine church architecture: the basilica and the circular temple. The latter was known in pagan times, but acquired greater complexity in C4 when circular clerestoreyed domed structures (as at Santa Costanza, Rome) were developed first as tombs, then centrally planned martyria (commemorative or pilgrimage shrines). It is clear that the martyrium was planned in a different way from an ordinary church, and from C4 martyria were known to have been constructed as octagons with radiating arms to produce cruciform plans.
The basilican type of church can be seen at Sant'Apollinare in Classe, Ravenna (534–49), where the clerestorey is carried on arcades set on rows of columns on rectangular pedestals and with curiously un-Classical capitals based on the Composite type. Above the abaci are blocks or dosserets from which the arches spring. Yet this building is essentially Italian, whereas San Vitale, also in Ravenna (c. 532–48), is very different: centrally planned, it has a clerestoreyed vaulted octagon carried on piers, a lower galleried aisle, and an apsidal chancel. Columns have block-like capitals, making the transition from circular shafts to square dosseret, and have virtually no connection with Classicism, while the bases are stepped and octagonal. San Vitale appears to have been a martyrium, and, architecturally, derives from the Church of Sts Sergios and Bacchos in Constantinople (c. 525–c.536), which has a clearstoreyed octagon. The beautiful lace-like capitals in Sts Sergios and Bacchos have only a suggestion of the Classical about them.
The great achievement of Byzantine architecture was the huge Church of Hagia Sophia (Holy Wisdom) in Constantinople (c. 532–7), designed by the scientists and mathematicians Anthemios of Tralles and Isidorus of Miletus. Various themes that were familiar at the time were synthesized and combined in one design, and it was rather as though the basic form of Sts Sergios and Bacchos had been cut in two, greatly inflated, and built on either side of a gigantic square space covered with a low saucer-dome carried on pendentives. Such a huge dome on a square space and constructed thus was unprecedented, and the complete synthesis of the basilican and centralized plan can be found in that great building. The church's interior was enriched with a skin of coloured marbles, porphyry, and other stones, while the vaults and domes were covered with the mosaics that were such a glorious feature of Byzantine churches.