Istanbul's history dates back to 633 B.C. when Doric settlers from Megara founded a small, commercial colony here that became known as Byzantine. Two major constraints dictated the location of ancient cities: topography and strategic considerations. The site of this new town was located at the tip of a peninsula that commanded three waterways. With the formal establishment of the polis, a city wall measuring five kilometers in length and having twenty-seven towers was built as protection. Within the walls, a hill within the walls was selected as its acropolis. This was the first of the city's eventual seven hills - apparently a topographical "must" for legendary ancient cities.