The cathedral of the bishopric of Mariana, called Canonica, is dedicated to the Virgin of the Assumption and was built at the beginning of the 12th century on the ruins of a paleochristian basilica. Consecrated in 1119 by the archbishop Landolfe of Pisa, it was flanked, probably until the 13th century, by an episcopal building.

It was gradually abandoned by the bishops in the early Middle Ages because of the insecurity of the shores, as attested by a document of the late fifteenth century.
This building with a basilica-shaped plan, with its great limestone and cipolin of Brando apparatus, consists of a central ship and two lateral ones covered with a wooden frame - except the span on the right of the choir, characterized by a semicircular-arched vault since the 17th century.
The central ship is extended by a semicircular apse with a cul-de-four vault. The building is covered with a gray granite roof. The semicircular arch surmounting the bare tympanum of the western gate is carved with griffins and quadrupeds in high relief. The apse is decorated in a sober style with blind arcatures falling alternatively on modillions or on pilasters.