Byzantium was a Greek city, according to legend first settled by Megarians and Argives under (the probably fictional) Byzas about 657 B.C.. In 196 A.D. it became nearly completely destroyed by Septimus Severus. Later, he rebuild large parts of the town and renamed it Augusta Antonia. The city was redounded by the Roman Emperor Constantine the Great in 330 A.D. with special ceremonies, some of which were in his newly adopted Christian religion but most of which were in the traditional Roman religion therefore reinforcing the political link with Rome, and became shortly afterwards the new capital of the Eastern Roman Emperor; renamed Constantinople after the Emperor.
Byzantium remained the capital of the eastern part of the empire when the empire was divided into two halves in 395 A.D., and whereas the western half fell to Barbarian invasions in the space of less than a century, the eastern half survived through the entire Medieval period. Initially the center of Eastern Orthodox Christianity (Byzantine Empire), it was sacked by the Western Christian forces of the Fourth Crusade in 1204 and wasn't regained by the Byzantines, under Emperor Michael VIII Palaeologous, until 1261. After this it grow weaker and smaller.
Constantinople finally fell to Ottoman forces on Tuesday 29 May 1453 A.D. It is today the Turkish city of Istanbul.