In 1176, a legend associated with the church arose and spread, that held miraculously the bodies of the martyrs held in this church flew as doves to the field of Battle of Legnano, and landed on the city of Milan's Carroccio, (a ceremonial war wagon).

When the building was modified between the 12th and 13th centuries, giving it the present Romanesque appearance, the original walls were preserved to a height of 22 meters.

A great multitude came to watch vigil, and the origins of Peter's cult began, as people started to report miraculous occurrences.

The apse vault is decorated by what is considered Ambrogio da Fossano's masterwork, the Incoronation of Mary.

Also on the left of the apse is the entrance to the small sacellum dedicated to the Martyrs of Anaunia, not before the end of the fourth century, as in a passage in Maximus of Turin's Sermo 81[2] Maximus designates himself a witness of the martyrdom of three missionary priests in 397 at Anaunia in the Rhaetian Alps.