St. Gereon Church: Exterior view from the west, showing the porch and great decagonal-shaped dome

The church's history can be traced back to a Late Antique mausoleum built ca. 360 in a cemetery outside the town walls. Saint Gereon was a Roman solider and martyr. The original building had an oval ground-plan with an entrance hall to the west. From 1219 to 1227 the original structure was transformed into a centralized decagonal building based on the old ground-plan. The vaulting of the eight niches remained untouched, but the elevation was completely reshaped in the style of northern French Early Gothic. The east exterior, an extension of choir ca. 1150, has two square east towers and an apse.