The Aurelian Walls are the city walls built between 271 AD and 275 AD in Rome, during the reign of the Roman Emperor Aurelian. The walls take up a circular perimeter of roughly 19km, the longest in existence. The walls were designed to have a height of 8m, which today has doubled to 16m, with a depths of (thickness) 3.5m. Originally, the Aurelian Walls consisted of 383 towers, 7020 merlons, 5 minor gates, 116 toilets, 2066 great windows, 18 main gates. Typically, the distance between a tower and another measured 100 Roman Feet (equivalent to 29.6 cm).