According to a document dated 1297, this sacred building was the castle chapel and was dedicated to St. Peter. The oratory changed names with the arrival of the relics of the presumed Saint Costanzo, preserved inside a valuable reliquary.
Following 17th century expansion work, the oratory took on a typically baroque appearance. It was demolished in 1888 following the earthquake and rebuilt in a different orientation. However, the altar, the fruit of a long construction project, entrusted firstly to Daniele Solaro (1634- 1699) and then afterwards, in the 18th century, to Ambrogio Franzone, is preserved.
On the alter may be seen the painting depicting the Martyrdom of Saint Costanzo by the Genoese painter Domenico Piola (1628-1703). A locally made, 15th century crucifix is also preserved.