The most substantial changes made ??during the Baroque period (1663), such as the raising of the stone floor, the staircase access to the choir and the construction of the vault on the nave, although a change in the basilica in Latin cross plan, however, show a clear cohesive and coherent mix.
The interior has three naves with a particularly pronounced inclination of the floor. Bas-reliefs of various ages are set along the walls. Frescoes from different eras adorn the interior walls: the oldest are those left of the choir arch (XIII and XIV century), while the apse and choir (Doctors of the Church, scenes from the life of St. Peter, the strips and cherubs' heads, bundles of fruit, chess boards) are probably the mid-seventeenth century. From the same period are scenes from the life of St. Carlo Borromeo, above the side door on the right wall, attributed to Alessandro Gorla. Near the entrance, on the right, is the Chapel of the Rosary was built in 1600 by Cav. G. B. Pellanda.
From the Way of the Cross Church, restored in the years 1991-97, which leads to the oratory of S. Petronilla.