It is annexed to the city's Cathedral, but stands as an independent building on the Piazza del Comune. Begun in 1167, it is characterized by an octagonal plan, a reference to the cult of St. Ambrose of Milan, symbolizing the Eighth Day of Resurrection and, hence, the Baptism. The edifice mixes Romanesque and Lombard Gothic styles, the latter evident in the preference for bare brickwork walls. The upper part was rebuilt (1554-1556) by Francesco Dattaro.