The cathedral measures 102 x 40 m. At the west end two squat, square towers project beyond the aisle walls and flank a portico, built in 1770. The exterior of the east end [chevet] is covered in polychrome decoration of inlaid stone, creating an elaborate pattern of intersecting arcades and roundels . Inside, the nave appears unusually wide, with large pointed arcades supported by antique columns and capitals opening on to the aisles. The crossing with its massive, wall-like piers is almost square, and it is surmounted by a lantern pierced by large windows. The slightly projecting transepts lead to deep apsidal chapels. The dominating feature of the building is the main apse, framed by the receding sequence of arches in the chancel. The apses and presbyteries are spanned by pointed barrel vaults; other parts are covered by a timber roof. The large expanses of wall surface seem to have been designed with the mosaic decoration in mind.