Surrounded on three sides by water, huge walls across the peninsula completed the city defenses. In the 5th century AD, under the Byzantine emperor Theodosius II, four miles of city walls were built, stretching from the Golden Horn to the Sea of Marmara. The fortifications were a series of stone towers, connected with a line of inner and outer walls, and a moat. From the towers, the defenders could catapult their favorite weapon at the attackers, the famous "Greek fire."

These walls held for a thousand years, as Constantinople was besieged by Attila the Hun, Arabs, Russians, Seljuk Turks and Crusaders, until 1453. The Ottoman Turks, led by Mehmet the Conquerer, bombarded the walls with enormous cannons until the walls were breached, and the invaders poured into the city.