In 1028[1] the present cathedral was founded by Bishop Jacopo the Bavarian to replace it, as he wished it to be inside the city walls.

The cathedral is built on the basilica floorplan with a nave and two aisles separated by stone columns which have capitals decorated with figures and animals.

In the presbytery is a polyptych of the Three stories of Saint Nicholas, by Bicci di Lorenzo, commissioned in 1450.

The decoration includes late Gothic medallions on the vault, while on the left wall is a cycle of the Stories of St. Romulus by the school of Domenico Ghirlandaio.

The bishops' funerary chapel has a 13th-century icon attributed to one "Master of Bigallo", portraying the Madonna and Child (1215–1220).