Greeks around 657 BCE founded Byzantium. It was significant because of its strategic location that connected it to east and also in terms of defence. it could only be attacked from the west as rest sides were covered with sea.
Because if its prominent location and fertile land, it was marred with wars in the early times. It was a centre of Persian wars between Greeks and Persians. Persians were hell bent on conquering Greece and city kept changing hands between Persians and Greeks. It was later conquered by Spartans who made peace with Persians (Persian king Xerxes) and provided their full support to Persians in exchange of letting them govern Byzantium.