The cathedral was built between 1574 and 1592 on the grounds of the former church of San Quirino. The facade was added in 1880 by the architect Nicolò Bruno in a neoclassical style. The altar from the mid-18th century and works by famous painters are remarkable. The urn of the first bishop and patron saint of Tortona San Marziano Martyr is in the presbytery. The order of the Augustinian sisters Annunziata was once at home here. One side of the Annunziata cloister from the 16th century has been preserved.
The Bishop’s Palace is located next to the cathedral. Bishop Cesare Gambara had the palazzo built in the style of his hometown Brescia. Bishop Carlo Settala connected the palace to the cathedral in 1659. There are important paintings in the palace, including a triptych The Madonna with the Child.