The imposing baronial castle of Fondi
is unquestionably the symbol of the town. It is a very rare case in Europe of a fortress built in a flat position. It consists of two bodies: the stronghold, 39 metres long and divided into two floors, with three cylindrical corner towers with defensive functions, and the donjon, 33 metres high in all and formed by a massive square base surmounted by a cylindrical tower.
The beginning of its construction dates back to the first half of the 14th century. The castle was in fact an integral part of the medieval walls, erected in the decade 1319-1329 by Roffredo III Caetani in parallel with the ancient one.
The construction of the fortress was continued in the following decades by other members of the Caetani family. That the building attained its final appearance before the 15th century is supported by the triptych by Cristoforo Scacco, preserved in the cathedral, which depicts St Onorato holding a model of the castle coinciding with its current appearance.
In the 19th century, the beautiful Ghibelline battlements were removed because they were unsafe. From 1861 to 1931 it was used as a district prison. In some rooms before the 20th century restoration, graffiti made by prisoners could still be seen.
During the World War, aerial bombardment destroyed the two northern towers, which were rebuilt after the war. Today, the rooms of the castle house the civic museum, with exhibits ranging from prehistoric to modern times.