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 “converted” in the early 13th century into the shape of a decagon (decagon) with four ancient conches in both the north and south; this late Romanesque central building is unique in its kind north of the Alps. The central ribbed vault, which closes the room divided into four floors, can be referred to as the largest dome construction of its time (1227). It reaches a height of 34.55 meters at the apex and measures 21 meters and 16.90 meters in diameter. The windows of the top wall zone, grouped like a tracery, follow early Gothic models from France.
To the west of the central room is the porch in Gothic forms, in which the late antique narthex lives on. To the east of the Decagon is the two-bay long choir with the choir square above the crypt, to which the two east towers and the apse are connected. The Staufen floor choir, richly designed from the outside, has seven blind arcades with three windows.
The three lower floors of the towers end with the apse apex, followed by two more with mock windows and a fifth floor with two spacious double arcade windows. The tower roofs are richly folded. The towers themselves have a close visual relationship with the decagon, which also emphasizes the singular character of the basilica in the distant view.
Source: Wikipedia