Documented since 1029, the Abbey of San Pietro was founded by a Benedictine community. The architectural lines of the church date back to the second half of the twelfth century and were completed in 1253, when Innocent IV consecrated it. The top of the façade was renewed in 1268; originally it ended with a tympanum, which was demolished in the damage caused by the earthquake of 1832.
It is divided into three naves by strong pillars with three entrance portals, three rose windows and rectangular pillars; the central part is opened by pointed arches on which lie the covered wooden beams, the sides of the vault and ceiling are in in brick. Above the altar – which holds the remains of the first bishop of Assisi Vittorino – is a dome formed by 31 concentric rows of projecting blocks. A left apse chapel retains some thirteenth-century frescoes, depicting hunting scenes and figures of saints. In the left transept are frescoes of the XIV century.