The eastern half of the Roman Empire, based in Byzantium (later Constantinople, now Istanbul), an ancient Greek settlement on the European side of the Bosporus. It was inaugurated in AD 330 by the Emperor Constantine I who transferred the capital of the Roman Empire to Byzantium. The empire survived the collapse of the Western empire until overrun by the Ottoman Turks in 1453. Originally a Greek colony at the entrance to the Black Sea, a typical Roman town was then laid out over it. Remains of the imperial palace lie south of the former Greek city nucleus. The land walls, giving the city an area greater than that of Rome, were built by Theodosius II (408-450 AD) and are among the best-preserved ancient fortifications anywhere. In the 7th century BC Dorian Greeks founded the settlement of Byzantium on a trapezoidal promontory on the European side of the Bosporus channel which leads from the Mediterranean to the Black Sea and separates Europe from Asia. Septimus Severus (193-211 AD) was responsible for restoring the city, re-walling it and beginning the construction of the limestone racecourse, the Hippodrome. In 368 AD, Valens raised his still impressive aqueduct. In 413 Theodosius II built the colossal surviving walls of stone and brick-faced concrete, with 96 variously shaped towers, and the principal entrance at the Golden Gate. The Eastern Christian empire preserved much of Greek and Roman culture and introduced eastern ideas to the west. Byzantium was essentially a Christian church state, preserving its religion against the onslaught of Islam, despite the Arab encroachments on Palestine, Syria, and northern Africa during the 6th-7th centuries AD. The Byzantine period is the time, about the 6th-12th centuries AD, when its style of architecture and art developed. Byzantine architecture is noted for its Christian places of worship and introduced the cupola, or dome, an almost square ground plan in place of the long aisles of the Roman church, and piers instead of columns. The apse always formed part of Byzantine buildings, which were richly decorated, and contained much marble. St. Sophia (532-537), St. Mark's (Venice, 977) and the Cathedral of Aix-la-Chapelle (796-804) are of pure Byzantine style. Byzantine painting preceded and foreshadowed the Renaissance of art in Italy. Mosaics are perhaps the supreme achievement of Byzantine art.