The construction of the Euryalus Castle (from greek euryelos: nail with large base) is due to the military genius of Dionysius. After the war they saw Syracuse besieged by the Athenians (415-413 BC) and that showed the weakness of the defensive sector, in the years between 402 and 397 the tyrant shut the city within a city walls long 27 kilometers that had its spearhead inside the castle. It was a feat that ancient historians have left us with the tone of admiration; in any case we are dealing with a very cyclopean work (15.000 sq.) for the times. After the Roman conquest of the city (212 BC), the great Euryalus was amended several times, until Byzantine when it was rebuilt using a part of the materials taken from other parts ruined. The fort was preceded by three large ditches: now are visible the second and third; the first is buried and was at the height of the entrance ticket. The squared blocks that are seen in the second moat belong to the perimeter structures. From the third moat, which was at the foot of the fort, there is a branched network of tunnels and passages. Also in this third moat there was a rivellino connected to the castle by a drawbridge. The fort was protected in Greek times by a triangular spur (which are some collapsed remains), then replaced by five towers that reached the height of 15 meters; it is likely that on their top there had been housed catapults. From the plateau it can still see the northern section of the wall where was a door (work pincer).