Found this great map/model of the city of Constantinople as it was in the Byzantine period. Surrounded by water this was a great defensible location and also open to trade through ease of access to the Black Sea and the Mediterranean. The city's origin was as a Greek city founded in 657 BC by colonists from Megara (near Athens, Greece). It became capital of the Roman Empire under Constantine in 324, thus the name change to Constantinople. The city continued as capital of the Byzantine Empire until 1453 when it was conquered by Fatih Mehmed II and his Ottoman army thus becoming the capital of the Ottoman Empire (Mehmed was 21 at the time). The name Istanbul was maintained after the founding of the Turkish Republic in 1923.

Note the circular forum of Constantine with a central honorific column (one of the only remnants still extant from the Constantinian period in the city). Further back, note that there are two city circuit walls (the inner, first wall, is much less well preserved). The first wall was built in the fourth century when Constantine expanded the city from its more humble origins and the second, further out, was build in the early fifth century under the emperor Theodosius II. The entire area within the walls was not developed but served as fortification for fields and underground cisterns. These fortifications, still visible today, protected the city until May 29, 1453.