Dating back to the years 837-838, the Castle of Monte Sant’Angelo stands in the upper part of the town and was home to Frederick II of Swabia, the historical emperor and commander, responsible for the construction of the Treasure Room. During the rule of the Angevins, it essentially became a state prison; the Swabian princess Filippa of Antioch and Giovanna I of Naples were imprisoned here. The latter probably died here.
Its present-day appearance is mainly the work of the military engineer Francesco di Giorgio Martini, who had to work on the fortification between 1491 and 1497 to prevent damage caused by the discovery of firearms.