A mighty fortification work, of which evocative remains remain, closed the only natural access to the hill where the vast plateau overlooking the ancient city culminates, the Epipolis, which the brilliant defensive strategy of Dionysius surrounded entirely with one of the most extraordinary city walls of the Greek world, more than 27 km long, built at the beginning of the 4th century. BC The vast fortress, which underwent various modifications over time, especially by Agatocle and Ierone II, is preceded by three ditches, with a powerful work advanced between the second and third moats; the latter was originally bypassed by a connection of which the three mighty support pillars remain; the front of the keep is protected by five towers, originally 15 m high. A complex network of tunnels opens at the bottom of the third moat, connecting different areas of the fortress and this one with the outside. Three large cisterns ensured the water supply. A large pincer door with three openings opens to the north of the keep, protected by advanced works. To the north and south, the fortress connects to the Dionysian walls that surround the plateau.