The Castello Aragonese occupies a small rocky island at the point where the navigation canal separates the Mar Grande from the Mar Piccolo. The current fortification was substantially rebuilt by Ferdinand I of Aragon in the late 15th century after Ottoman forces devastated the city, and the four cylindrical towers were designed to be able to absorb cannon fire, an innovation in European defensive architecture at the time. The design was attributed to the Sienese engineer Francesco di Giorgio Martini, who worked extensively for the Aragonese crown across southern Italy.
The castle is still an active Italian Navy installation, which means visits are by guided tour only and there are days when access is restricted altogether. The interior is less spectacular than the exterior warrants, but the views from the battlements across both the Mar Grande and the Mar Piccolo give you a sense of why this particular point was fortified in the first place — it controls everything entering or leaving the sheltered inner sea. The drawbridge, moat, and entry gate remain in their medieval configuration.