A mighty fortification work, of which suggestive remains remain, closed the only natural access to the hill where the vast plateau overlooking the ancient city culminates, the Epipoli, which the ingenious defensive strategy of Dionysius entirely surrounded by 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 Agathocles and Hieron II, is preceded by three moats, with a powerful advanced work 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 three-opening pincer door 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.