The Castello di Lombardìa (Lombardy Castle), perhaps the most important example of military architecture in Sicily. It was created in very old times by Sicanians, rebuilt by Frederick II of Sicily and restructured under Frederick II of Aragon. The castle, which bears this name because of the garrison of Lombard troops that defended it in Norman times, has an irregular layout which once comprised 20 towers: of the six remaining, the Torre Pisana is the best preserved one. It has Guelph merlons. The castle was divided into three different spaces separated by walls: the first courtyard is home to a renowned outdoor lyric theater in which high quality shows are performed; the second one houses a large green park, while in the third courtyard it is possible to see the vestiges of royal apartments, a bishop's chapel, medieval prisons and the Pisan Tower.
The Duomo (Cathedral), a noteworthy example of religious architecture in Sicily, built in the 14th century by queen Eleonora, Frederick III's wife, but largely renovated after the fire of 1446. The great Baroque facade, in yellow tufa-stone, is surmounted by a massive campanile with finely shaped decorative elements. The portal on the right side is from the 16th century, while the other is from the original 14th century edifice. The interior has a nave with two aisles separated by massive Corinthian columns, and three apses. The stucco decoration is from the 16th and 17th centuries. Art works include a 15th century crucifix panel painting, a canvas by Guglielmo Borremans, the presbytery paintings by Filippo Paladini (1613), a Baroque side portal. The cathedral's treasure is housed in the Alessi Museum, and has precious ornaments, the gold crown with diamonds "Crown of the Virgin," Byzantine icons, thousands of ancient coins and other collections.