The Cathedral, with Latin cross, was built in 1171 on the ruins of a Greek temple. It consists of a single nave with three central apses. The ongoing interventions and renovations have altered its original Romanesque architecture. The facade waa rebuilt several times, until the replacement of the wooden roof in 1600. The back, which is accessed by seven steps, it includes two chapels and the central altar: the left chapel, largely distorted by a remake of the year 1000, was erected by the family Gattinara; the central altar, in Baroque style, was commissioned by Bishops De Marco and Capreoli.