The walls and ceiling of the fine Romanesque interior are covered with frescoes of 1570, subdued in colour and well suited to the character of the building; those of the octagonal cupola representing the Assumption of the Virgin are by Correggio, but much restored.
John are by Correggio, and the arabesques on the vault of the nave by Anselmi.
As a republic its government was mainly in the hands of the Rossi, Pallavicino, Correggio and Sanvitale families.
In 1307 the city became a lordship for Giberto da Correggio, who laid the basis of its territorial power by conquering Reggio, Brescello and Gaustalla, and was made commander-in-chief of the Guelphs by Robert of Apulia.
The Correggio family never managed to keep possession of it for long, and in 1346 they sold it to the Visconti (who constructed a citadel, La Rocchetta, in 1356, of which some remains exist on the east bank of the river, while the later ate du Pont may be seen on the west bank), and from them it passed to the Sforza.