The original walls of the city were built in the 8th century BC when Byzantium was founded by Greek colonists from Megara. At the time the city consisted of an acropolis and little more. The colony was relatively unimportant by the Roman period, but Septimius Severus built a new set of walls during his reign, bringing more land into the city. When Constantine moved the capital of the empire to Byzantium, which he renamed Constantinople, he greatly expanded the walls of the city, incorporating even more territory.
In 412 the Byzantine emperor Theodosius II surrounded Constantinople with a wall that stretched 6,5 kilometers between the Sea of Marmara and the Golden Horn. Although named after Theodosius, they were actually constructed under the direction of Anthemius, Praetorian prefect of the Eastern Empire.
The walls were built of stone and brick in two lines of defense which adjoined a ditch 20 meters wide and 10 meters deep. The inner wall had was 5 meters thick and 12 meters high. It was strengthened with 96 unapproachable towers 18-20 meters tall. The thickness of the external wall was 2 meters, and its height was 8.5 meters. The wall contained the Belgrad, Silivrikapı, Mevlevihane, Topkapı, Edirnekapı, Eğrikapı and Yedikule gates. Yedikule Gate was also known as the "Porta Aurea" or "Golden Gate", and was the most magnificent, consisting of three archways. As the state entrance into the capital, the Golden Gate was architecturally splendid, and was built of large square blocks of polished marble fitted together without cement, and flanked by two great towers of the same material. Upon the gates were placed sculpted elephants.
The inner walls had massive square or hexagonal towers. The southern end of the walls, at the Sea of Marmara, was the location of the Marble Tower. There were ten gates, the four major ones being the Golden Gate at the southern end, the Selymbria Gate leading to the Forum of Constantine and Hagia Sophia, the Adrianople Gate connecting Constantinople's Mese (the main street) with the city of Adrianople and the Via Egnatia, and the Blachernae Gate in the north.
In 439 the first sea walls were built. In 447 the wall was damaged by an earthquake, but was quickly restored in time to stop Attila's army. In the northwestern corner of the city, in the suburb of Blachernae, the Theodosian walls were relatively weaker, but were expanded by Leo I. The northern coast of the city, along the Golden Horn, also had a separate set of walls.
These walls were almost inpenetrable, saving the city during sieges from the Bulgars, Arabs, and Rus', among others. The first attackers to break through the walls were the knights of the Fourth Crusade, who managed to scale the lower sea walls and also break the Wall of Leo in the northwestern Blachernae section of the city.
The second and final time the walls were breached was during the Fall of Constantinople in 1453, by the Ottomans; however, they did not break through by force, but entered through the Kerkoporta Gate, which happened to be open, apparently accidentally but possibly through treachery. A large plaque today marks the spot where the Ottomans entered.
Under the Ottomans, the Golden Gate, known to them as Yedikule ("the Seven Towers"), was used as a prison. Many parts of the walls are still standing today and are a testament to the extraordinary longevity of the empire. They have been damaged by recent earthquakes, but many sections have also been restored. The wall runs through the suburbs of modern Istanbul, with a belt of parkland flanking their course. The walls are pierced at intervals by modern roads leading westwards out of the city.