The Byzantine Empire began with the founding of the city of Constantinople by the Emperor Constantine the Great in 330 CE. By the 12th century Constantinople was the largest and richest city in Europe. Byzantium maintained its glory until the sacking of of the city by the forces of the Fourth Crusade in 1204 CE. After this the Byzantine Empire existed only in fragments. The Ottoman conquest of the Constantinople in 1453 CE ended the empire, replacing Eastern Christianity with Islam.
The art and architecture of the Byzantine Empire is justly famous. Buildings such as the Hagia Sophia taught the world the use of space; the mosaics of Ravenna, Venice and Palermo gave the Christian religion its iconography. This is the Byzantium of Yeats’ poem Sailing to Byzantium (1927)
O sages standing in God’s holy fire
As in the gold mosaic of a wall,
Come from the holy fire, perne in a gyre
And be the singing-masters of my soul.