[3] St. John's Church is predominantly Carolingian in style, but later exterior additions over many centuries have resulted in the appearance of various architectural influences seen today.

Knöchlein speculates that as the predecessors have also been made of stone, which is uncommon for the post-Roman period, leads to the conclusion that the early Christians in Mainz worshipped first in either a civilian building near the Forum or on the foundations of an ancient temple.

[8] One fact is that the ground level has risen by 2.60 meters since Hattos times comprising debris of solid masonry.

[9] When the scientists of the Institut für Europäische Kunstgeschichte of the Heidelberg University were scratching the plaster of the wall, they encountered in some places still in nine meters height tuff masonry from the 6th or 7th century.

The walls of the nave are at the top of the clerestory broken by four arched windows, including four arcades open each (now closed) in the former aisles (now parish rooms).