The imposing and elegant circle of walls is the monument, which best represents the city and its history. Because of the enlargement of the urban centre over the years, it was necessary to extend the city walls in different times and ways.

Three are indeed the circles of walls, which were built throughout the ages. The first quadrangular-shaped one dates back to the 3rd or 2nd century b. C. The nine meters high structure was made up of layers of regular stones. Four gates were built at the ends of the cardo (from the South to the North) and of the decuman (from the West to the East). The current Via della Rosa (where are the only one remains of that circle of walls), Via dell'Angelo Custode, Via Mordini, Via degli Asili, Via S. Giorgio, Via Galli Tassi, Via S. Domenico, Via Cittadella and Corso Garibaldi marked the boundaries of the area inside the Roman walls.

But we don't know very much further about that circle of walls. Surely it was restored and extended in several stages, but as the town began to develop unevenly in the Middle Ages, the construction of a new circle of walls, dating back to the 13th century, was necessary. However it seems some pieces of walls had been built before to protect the villages on the outskirts, which were more exposed to the outside assaults. S. Maria Forisportam, S. Pietro Somaldi and S. Frediano are only some of the examples of the villages, which were included inside the new city walls. The new fortifications were three-meters higher. The circle of walls was made up of squared stones and supported by cylindrical towers. It had also four gates, called Porta S. Pietro, Porta S. Donato, Porta dei Santi Gervasio e Protasio and Porta dei Borghi (o di S. Maria).