Carreum suffered a drastic demographic decline and a downsizing of the town from the end of the second century AD, probably due to the hydrogeological instability that caused the landslide of the unstable hill of San Giorgio with the consequent abandonment of large sections of the city.
In the area of the future baptistery, on the site of a first imperial domus, late-ancient walls arose, perhaps connected to an early Christian cult building with an adjoining cemetery, as attested by an epigraph from the fifth century.
In the second half of the 6th century, nuclei of Longobards settled on what remained of the Roman city, as evidenced by some precarious structures and the remains of a church for funerary purposes.