The exterior of the Cathedral of Santa Maria Assunta consists of a wide three-bay pronaos supported by twelve columns with carryover capitals. The Romanesque-style main portal, similar to that of the church of St. Menna, has on either side two pairs of columns with Corinthian-style capitals. Baroque stuccoes were added in the 17th century. The interior has a Latin cross plan with three naves, transept, dome and chancel. In the side chapels are valuable works of art framed by Baroque stucco and having polychrome marble altars.
The nave ceiling is in painted and gilded wooden boards, a 19th-century work, while the high altar, set back from its original position, is in inlaid polychrome marble. Beneath the transept is the Romanesque crypt, where the numerous vaults rest on carryover columns while several traces of frescoes are visible on the walls.