The Northern Gate, one of the main gates of the city, is well preserved. Outside it is a sophisticated defence system once thought to date from the time of Hermocrates, but probably constucted by Agathocles in 307-306 B.C. The fortifications include three semicircular towers, and a second line of walls 5 m outside the earlier ones which were reinforced after their destruction by Hannibal in 409 B.C. using material from the acropolis including capitals.