The origins of the cathedral, dedicated to St Mary and St Marcellus the Pope, are lost in the Middle Ages when the town spread out into the valley. In the 18th century the church was enlarged (the plan became basilica-like, the façade was limited by pilasters and leaning columns) and embellished with decorations that highlight an elegant Baroque style.
Of particular interest is the pipe organ. According to some scholars it is the work of Cesare II Catarinozzi, the "Stradivarius of the organ". The body of Saint Constance the martyr is kept in a glass urn under the high altar.