Erected between 1284 and 1288, during the lordship of Ugolino della Gherardesca Count of Donoratico, it rose in 1503 to the role of cathedral, still the only one dedicated to the Saint of Assisi. What remains of the original building represents evidence of the transition from Romanesque to Gothic in an island key. The facade is divided into two orders by a molded cornice: in the lower opens the architraved portal surmounted by a round arch; in the upper an oculus is narrowed by two pointed arches, while a series of thirteen trefoil arches follows the course of the gabled roof. On the right side stands the bell tower. The interior, extensively remodeled since the 16th century, has a Latin cross plan with a single nave and side chapels. Pointed arches divide the hall into four bays covered by star vaults.
