From 1488 to 1492 local hands directed by the florentine architects Francesco di Giovanni called “Il Francione” and Domenico di Francesco called “Il Capitano” built the fortress of Cittadella, called Firmafede, on the east side of the town, on the ruins of the previous fortress “Firmafede” built by the Pisans in 1429.

In 1494, with the coming of Charles VIII, the fortress was widened. Following the fortunes of the town The Bank of St. George arranged for the enlargement of the whole defending structure futher defining the course of the moats.

In the 19th century there was the last intervention with the superfetation of a part of the central body, to create new rooms to use as prison.
The plan is built on the juxtaposition of two big rectangles, with cylindric towers on the tops, to form two equal fields, divided by the separating hill.

The donjon is in the inside body of the building, because if there had been an attack it could have been the second defending barrier.
The structure of the circular tower and of the donjon, not very suitable for a grazing defence, reveal a quite old-fashioned solution for times in which new tecniques had been developing.