The building that connects the cathedral to the Palazzo dei Papi is the third most important in the city. It is a museum housing items that once belonged to the popes, as well as archaeological finds from the Etruscan, Roman, and Medieval periods.

Among the artworks, visitors can see the special piece Madonna della Carbonara, which came from Rome despite its Byzantine style, an impressive Etruscan sarcophagus, and original papal garments.