Already mentioned in 970, the actual Romanesque-Gothic Benedictine church dates back to the 13th century and is contemporary of the Basilica of St. Francis.
As a matter of fact, Pope Innocent IV consecrated it in 1254, together with St. Francis and St. Rufinus.
Pillars and a cornice divide the front into two orders of three rectangular panels each: three rose-windows correspond to the three portals below. Originally, as all Benedictine churches, the front ended with a tympanon, subsequently demolished after the damages due to an earthquake in the 19th century.
The interior has three aisles, a short transept and originally three apses. The central nave, not having any windows, is lightened exclusively by the central rose-window. The ceiling of the nave is made of wooden beams, while the aisles have flat barrel vaults.
Five stairs lead to the barrel-vaulted transept under which lies the crypt. The central apse is semicircular, the left rectangular, while the right has been walled. On the inner wall of the front and along the sides of the presbitery, Gothic sepulchral monuments of the 14th century. In the left arm of the transept opens a chapel of the 14th century with frescoes of the same period.