The original city was called Byzantium. It was founded by Greek colonists from Megara around 657 BC.The name was changed to Constantinople in the 4th century AD.
Although becoming an ally of the
Roman Empire and in many ways becoming very Romanized, Byzantium remained fairly independent, acting as a stopping off point for Roman armies on their way to Asia Minor. The fishing, agriculture, and tributes from ships passing through the Strait made it a valuable source of income for
Rome. In 192 AD after Emperor
Commodus was assassinated, a war emerged over who would succeed him. When the city refused to support Septimus Severus, supporting Pescennius Niger of
Syria instead, the future emperor laid siege and destroyed the city. He would later regret his actions --- due to the influence of his son
Caracalla ---and rebuild it,
When Emperor
Diocletian divided the Roman Empire into his tetrachy (rule by four), Byzantium fell into the eastern half, ruled by Diocletian. The Emperor
Constantine came into power in the western half in 312 AD and would soon reunite the empire when he defeated Licinius at the Battle of Chrysopolis in 324 AD. He would build his new capital on the site of ancient Byzantium, New Rome; it would become the cultural and economic center of the east. Upon Constantine’s death in 337 CE, the city would be renamed
Constantinople in his honor. Although the city maintained it role as an important part of the
Byzantine Empire, it would be invaded and captured by the Ottoman Turks In 1453. For further information refer to the definition on Constantinople.
Here is a good short history of Byzantium:
https://www.ancient.eu/Byzantium/