Originally founded by Greeks 657 BC. For nearly sixteen centuries following its reestablishment as Constantinople in 330 AD, it served as the capital of four empires: the Roman Empire (330–395), the Byzantine Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of the last caliphate.
Impressive history - see here for more
Remember - Roman Republic - 509 BC–27 BC
Roman Empire - 27 BC–AD 476 (West), 330–1453 (East)