Adjacent to the baptistery is the Antiquarium, an exhibition space created in the old sacristy (ground floor) and the “custodian’s house” (first floor).

The rich collection of stone artifacts, uncovered during archaeological excavations, testifies not only to the long history of the monument itself but also to that of the structures that preceded it.

Of particular value are: a Lombard columnlet dating back to an altar from the 8th or 9th century, bearing a dedication from the priest Magnoald; a Roman inscription concerning an Augustal priest; and several late antique funerary inscriptions.