The medieval town developed around the Late Romanesque minster (11th-13th century), formerly a monastic church that can be traced back to an Early Christian chapel ("cella memoriae", ca. 300) dedicated to the martyrs, Saints Cassius and Florentius. The minster has a typically Rhineland Romanesque choir with square towers decorated with Lombard bands, flanking a galleried apse. The four-bay aisled nave has a three-storey elevation. The cloisters (1126-1189) are noteworthy. At one point the church served as the cathedral for the Archbishopric of Cologne. However, the Minster is now a Papal basilica.