Walls of Istanbul are the reason Byzantine Empire survived more than a thousand years. It was the most effective defense system with the length of 22 km. The city was surrounded by the walls which were renewed four times since the founding of the city. Firstly, it was built by Greek colonists from Megara in the 7th century B.C. These walls were demolished and the second zone of the walls was built by Roman Emperor Septimius Severus (193-211) who took over the city in 196 A.D. The first two walls were covering acropolis and settlements around Seraglio Point (Sarayburnu). The third wall zone was built by Constantine the Great (306-337) who moved capital of the Roman Empire to Istanbul. These walls, starting from Marmara Sea, going to the Golden Horn from Aksaray district on land, surround the city by following the coast.
The fourth wall which exists today was built during the period of Theodosius II (408-450) expanding twice. There are two rows of walls behind a deep ditch. It’s a unique defense system of Romans with 108 towers on low walls and 96 towers on high walls. Land walls connecting Yedikule in Marmara shore and Ayvansaray in Golden Horn shore are 6 km long. Sea walls in Marmara Sea and Golden Horn are parallel to the shore. Marmara walls are 8.5 km, and Golden Horn walls are 5 km long.