Constantinople has been known since 1930 as Istanbul, being nowadays the most populous city in Turkey and one of the most popular tourist destinations in the world.
After the triumph of Constantine I in his conflict with Licinius I, he became in the unique emperor of the entire roman world. He then decided to erect a large city in the eastern half of the Empire to replace the decadent and little pro-constantinian Rome (the Urbs had supported the usurper Maxentius a few years before) as capital of the Roman state. After studiyng a couple of possible sites, Constantine chose the place where the ancient greek city of Byzantium was located. The enclave, historically known as the "Golden Horn", was in fact very remarkable both from the tactical point of view (very easily defendable) as nautical (it possessed a magnificent port) and strategic (it dominated the Bosphorus strait or what is the same: the main crossroads between East and West since time immemorial).
Photo 1.- The immortal city-walls of Constantinople, erected in the time of Theodosius II.