Although it is a legend about the foundation of the city that has come down to us over the ages in various forms, it does not cast any real light on the facts surrounding the initial foundation of the city. According to a local legend which is comparatively much older than the others, the Thracian king Byzas, who was the son of the nymph Semestra, had married Phidaleia, daughter of Barbyzos, king of a region near to Istanbul; it was this woman who is said to have founded Byzantion, or Istanbul.

According to another legend lo, lover of Zeus, the chief of all the ancient Greek gods, turned herself into a cow to esacape the wrath of Hera, Zeus's vengeful wife. During her flight she gave birth to a daughter, Keroessa. on the banks of the Golden Horn. Keroessa was brought up by the nymph Semestra and in due course she gave birth to the son of the sea god Poseidon, whom she named Byzas. Byzas was brought up by the naiad Byzia. and he went on to found the city of Istanbul. It is possible to fit this legend in with the geography of Istanbul. On the other hand, the names Byzas and Keroessa are to be encountered in different forms in very old place names in Anatolia. This perhaps demonstrates that the legend originates in events that took place in the depths of Anatolia's history. According to legends originating in more recent times,(and one of these, born in the 1st century AD, is extremely well-known) Byzas had set out with the chief of a band of migrants from Megara in Greece. The oracle in the Temple of Apollo at Delphi had advised them to set up their new homeland in a place “facing the blind”. These migrants were said to have set up their first city on what is now Sarayburnu; this promontory lies opposite Kadikdy, formerly known as Khalkedon, which had been founded 17 (according to other sources 19 or 29) years earlier, and its founders had been accused of being blind because they had ignored the beauty of Istanbul. This last legend must be connected with the Greek migrations that took place between 750 and 550 BC and is certainly not related to the city's initial foundation. The only possibility is that during these migrations anew Greek city was founded in what is now Istanbul circa 660 BC, from which the present city developed.