St. Gereon is an outstanding example of late antique and high medieval architecture. The defining element of the building is a vaulted late antique oval building, which was “transformed” in the early 13th century into the shape of a decagon (decagon) with four antique conches each in the north and in the south; This late Romanesque central building obtained in this way is unique of its kind north of the Alps. The centralized ribbed vault that closes the room, which is divided into four floors, can be considered the largest dome construction of its time (1227). It reaches a height of 34.55 meters at its apex and measures 21 meters or 16.90 meters in diameter. The windows in the uppermost wall zone, grouped like tracery, follow early Gothic models from France.
To the west of the central room is the Gothic-style vestibule, in which the late antique narthex lives on. To the east of the decagon, above the crypt, is the two-bay long choir with the choir square, which is adjoined by the two east towers and the apse. The Staufian storey choir, which is richly designed from the outside, has seven arcades with three windows.
The three lower floors of the towers end with the apse apex, followed by two more with false windows and a fifth floor with two generous double arcade windows. The tower roofs are richly folded. The towers themselves have a close visual relationship to the Decagon, which emphasizes the unique character of the basilica even when viewed from a distance.
Source: Wikipedia