The former church of San Giovanni, with walls dating from the 5th–6th centuries, is the first Early Christian church of the Diocese of Asti. The history of the cathedrals is currently being revised due to evidence revealed by the major excavations being carried out in the area of the Episcopal Complex. Indeed, San Giovanni was connected by a quadriporticus to a second cathedral dedicated to Saint Mary, on whose 6th-century remains the current cathedral dedicated to the Virgin of the Assumption stands. The former church of San Giovanni now houses the Diocesan Museum and its nave and underlying crypt, connected by a southern subterranean nave, are open to visitors. The church of San Giovanni was rebuilt in the 9th century with a nave and two aisles. It had an exposed roof truss and the nave was illuminated from above by eleven small windows on each side. The church was probably destroyed during the fire of 1070, when the entire city was laid waste by Countess Adelaide of Susa. Around the mid-15th century, Archdeacon Giacomo De Gentis commissioned major work on the building, demolishing the apse and part of the 8th-century crypt, building the current façade with entrance on the eastern side, and closing up the original one on the west. The northern aisle of the church, situated on what is now Via Natta, had already collapsed in the 13th century, while the southern one was converted into the canons’ residence and subsequently demolished in the 18th century. The nave was covered with cross vaults and decorated in the mid-19th century. In the years that followed, what was left of the cathedral of San Giovanni was converted into a parish church and then into a small theatre, until the museum was inaugurated in the building in April 2010. Restoration work carried out in the 1990s stripped the plaster from the outer walls, revealing the architectural details of the 9th-century building and the 15th-century structure commissioned by De Gentis in the nave.