The cathedral is a three-aisled basilica, which combines late Romanesque and early Gothic elements. It has a narthex at the western end and a semi-circular choir with an ambulatory. The outside measures 54.5 m long, with a width of 35.4 m. The building has a complicated structure; seven spires rise from it. The number seven is a symbolic reference to the number of the sacraments. The tallest of these towers are on the western side and rise to a height of 37 m. They form the distinctive "". Such twin-tower facades are common in the Rheinland, for example at Xanten, and Koblenz.