Castello Ursino is a thirteenth-century fortress built by the Emperor Frederick II as part of his Sicilian defensive system. It is squat, square, and almost windowless — pure military architecture, with four corner towers and a central courtyard. The lava flow of 1669 stranded it inland, and the 1693 earthquake damaged it; but the basic structure is intact.
Today it houses the Museo Civico, the city museum, with a mixed collection of archaeological finds, paintings (including a small Antonello da Messina), and ceramics. The building itself is the main attraction. Entry is cheap and the rooftop terrace gives a different view of the centre.