The cathedral is in the square that bears the same name.

The original structure, built in the Romanesque style between the years 1187 and 1271, underwent numerous changes. Finally, in 1763, architect Cosimo Morelli completely renovated it because of unstable structures. The Church was consecrated in 1782.

The facade of the cathedral was remodelled in 1850.

The inside, which is in a neo-classic Latin cross style, has a central nave and two aisles, and is divided by Corinthian columns and clearly shows the symmetric and majestic style of Morelli.

The main feature is the presbytery, which is elevated in comparison to the level of the Basilica, and has two staircases leading up to it, while a central staircase goes down to the crypt which guards the remains of the Saints Cassiano, Pier Crisologo, Proietto, and Maurelio.

Special attention should be made to the craftsmanship of the wooden benches, the 16th century baptismal font in the first chapel on the left and the beautiful wooden crucifix of the 15th century above the alter.

There are numerous paintings of Angelo Gottarelli such as “L’esaltazione della croce”, which is in the second chapel on the right.

It is still possible to see the tombstone of Girolamo Riario, whose widow, Caterina Sforza, wanted to be buried in the Cathedral inside the Riario Chapel, which no longer exists.