The Walls of Jerusalem surround the old city of Jerusalem (approx. 1 km²). The walls were built between 1535 and 1538, when Jerusalem was part of the Ottoman Empire, by the order of Suleiman I. The length of the walls is 4,018 meters (2.4966 mi), their average height is 12 meters (39.37 feet) and the average thickness is 2.5 meters (8.2 feet). The walls contain 34 watchtowers and 8 gates. In 1981, The Jerusalem walls were added, along with the Old City of Jerusalem, to the UNESCO World Heritage Site List.