The first city walls of Istanbul were built during 413-477 by the Byzantine Emperor Thedosius II. They extend 6-7 km. starting from the Marble Tower on the Marmara shore up to the Golden Horn. The Yedikule Walls was built by Sultan Mehmet the Conqueror in the years 1457-1458. These walls contain 16 gates. The walls have a three stage defence consisting of the inner walls, outer walls and a trench. The inner walls are 3-4m thick and 13m high. The outer walls 15m away, are 2m. thick and 10m. high. In front of the outer walls, there is a trench. The Istanbul city walls are being restored within the framework of the UNESCO protection program.