The largest city of Turkey, in the northwest part of the country on both sides of the Bosporus at its entrance into the Sea of Marmara. Founded c. 660 BC as Byzantium, it was refounded in AD 330 by Constantine the Great, and under the name Constantinople it became the capital of the Eastern Roman, or Byzantine, Empire. The city was sacked by Crusaders in 1204 and taken by the Turks in 1453. Istanbul was chosen as the official name in 1930.