The famous Euryalus Fortress owes its name to the military genius of Dionysius. Its construction was a truly cyclopean undertaking, and took place between 402 and 397 BC within a circle of walls 27 kilometres long. This was prompted by events of war staged in Syracuse between 415 and 413 BC, which had highlighted its defensive weaknesses. In the 15,000 square metre area where the fortress stood, the keep was preceeded by three moats, the second and third of which are still visible, the first being buried under the level of the present-day ticket office entrance. In the second moat, you can still see the neatly squared blocks that once formed the perimeter structures. A network of galleries and tunnels also extended from the third moat at the foot of the keep, and a ravelin was connected to the castle by a drawbridge. In the Greek period, the keep was protected by a triangular outcrop (traces of which can still be found in the collapsed remains), which was later replaced by five towers that reached a height of 15 metres, on which catapults were probably placed. In the northern section of the wall, there was also a recessed gate (hornwork). With the Roman conquest of the city (212 BC), the large military complex of Euryalus was modified several times, until the Byzantine era, when part of the castle was rebuilt with rubble from other ruined areas.