During her flight, Io gave birth to a daughter, Keroessa, on the banks of the Golden Horn and after reassuming her original form, she gave birth to Zeus' son Epaphus beside the river Nile. Keroessa later borne a son to Poseidon, elder brother of Zeus. This son, Byzas the Megarian, in time became the founder of Byzantium.
According to legends originating in more recent times, (and one of these, born in the Ist century AD, is extremely well-known) Byzas leaded a band of migrants which left Megara to build a new city in Greece. Byzas had consulted the Oracle at Delphi to ask where to make his new city. The Oracle told him to find it "opposite the blind." At the time, he did not know what this meant. But when he came upon the Bosporus he realized what it meant: on the Asiatic shore was a Greek city, Chalcedon. It was they who must have been blind because they had not seen that obviously superior land was just a half mile away on the other side of the Bosporus. Byzas founded his city here in this "superior" land and named it Byzantion after himself. According to Greek legend, the Golden Horn derives its name from Keroessa, the mother of Byzas the Megarian, who named it after her.

Byzas. Ancient coin

It was mainly a trading city due to its strategic location at the Black Sea's only entrance. Byzantion later conquered Chalcedon, across the Bosporus. The city was taken by Darius 512 B.C. then it joined the Delian League in 476 B.C. The city was invaded once more, by the Spartans in the year 405 B.C.
Byzantium was taken over in 318 B.C. by one of the commanders of Alexander the Great, Antigonus. In the meantime, the Kingdom of Bithynia was founded on land situated between the Bosphorus and Sakarya. Invaded by the Galatians in 280 B.C. the city demanded protection from Rome.

Byzantine Constantinople

In AD 196, having razed the town for opposing him in a civil war, the Roman emperor Septimius Severus rebuilt it, naming it Augusta Antonina in honour of his son. In AD 330, when Constantine the Great dedicated the city as his capital, he called it New Rome. The coinage, nevertheless, continued to be stamped Byzantium until he ordered the substitution of Constantinopolis.

Constantinople in Nuremberg Chronicle

In the VIIth century, the citizens of Byzantium made the crescent moon as their state symbol, after an important victory. Byzantium was the first governing state to use the crescent moon as its national symbol. In 330 AD Constantine I added the Virgin Mary's star to the flag. Byzantium would then also be the first attested nation or empire to use the combination of the crescent moon and star together as an emblem. A variant of the city's emblem, depicted in blue and gold, was also used by Governor Isaac Komnenos of Cyprus (1184-1191).

The banner of Constantinople