The building underwent numerous alterations, ending with a profound restructuring in the eighteenth century. In 1763, due to serious problems, it was completely rebuilt according to designs from the architect Cosimo Morelli.
The facade of the church dates back to 1850, the result of the modification of the project by Cosimo Morelli.
The interior – neoclassical Latin cross style with three aisles divided by Corinthian pillars – clearly shows the imprint of symmetry and grandeur typical of Morelli. The main part is the presbytery raised above the floor of the Basilica, composed of two side staircases, while a third central staircase leads to the crypt that holds the marble sepulchers of SS. Cassiano, Pier Grisologo, Proietto and Maurelio.
Also noteworthy are the 18th century wooden choir, the baptismal font from the 16th century in the first chapel on the left and the second chapel on the right with a beautiful wooden the Crucified Christ from the 1400s.
At the quadreria level on the main altar, one can admire the “Martirio di san Cassiano” (Martyrdom of San Cassiano) by Pietro Tedeschi and numerous paintings by Angelo Gottarelli, among which “The Exaltation of the Cross” stands out, in the second chapel on the right from the entrance.
The funerary tomb of Girolamo Riario is still visible. Under the instruction of his widow, Caterina Sforza, it was buried in the Cathedral, in the Riario chapel, which is now no longer there.