The Walls of Constantinople () are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul in Turkey) since its founding as the new capital of the Roman Empire by Theodosius II. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger. They saved the city, and the Byzantine Empire with it, during sieges by the Avar-Sassanian coalition, Arabs, Rus', and Bulgars, among others. The advent of gunpowder siege cannons rendered the fortifications vulnerable, but cannon technology was not sufficiently advanced to capture the city on its own, and the walls could be repaired between reloading. Ultimately, the city fell from the sheer weight of numbers of the Ottoman forces on 29 May 1453 after a two-month siege.
The walls were largely maintained intact during most of the Ottoman period until sections began to be dismantled in the 19th century, as the city outgrew its medieval boundaries. Despite lack of maintenance, many parts of the walls survived and are still standing today. A large-scale restoration program has been underway since the 1980's.
Land walls
Walls of Greek and Roman Byzantium
According to tradition, the city was founded as Byzantium by Greek colonists from Megara, led by the eponymous Byzas, around 658 BC. The city then consisted of a small region around an acropolis located on the easternmost hill (corresponding to the modern site of the Topkapı Palace). According to the late Byzantine Patria of Constantinople, ancient Byzantium was enclosed by a small wall that began on the northern edge of the acropolis, extended west to the Tower of Eugenios, then went south and west towards the Strategion and the Baths of Achilles, continued south to the area known in Byzantine times as Chalkoprateia, and then turned, in the area of the Hagia Sophia, in a loop towards the northeast, crossed the regions known as Topoi and Arcadianae and reached the sea at the later quarter of Mangana. This wall was protected by 27 towers and had at least two landward gates, one which survived to become known as the Arch of Urbicius, and one where the Milion monument was later located. On the seaward side, the wall was much lower. Although the author of the Patria asserts that this wall dated to the time of Byzas, the French researcher Raymond Janin thinks it more likely that it reflects the situation after the city was rebuilt by the Spartan general Pausanias, who conquered the city in 479 BC. This wall is known to have been repaired, using tombstones, under the leadership of a certain Leo in 340 BC, against an attack by Philip II of Macedon.