The church can be visited
Sankt Stephan Church

The Catholic parish church of St. Stephan in Mainz is a house of worship on the city's highest elevation. It is the oldest Gothic hall church on the Middle Rhine and houses artworks.
History+
The church was founded in 990 by Archbishop Willigis, likely at the suggestion of Empress Theophanu, with the vision of creating a central place of prayer for the empire. Initially built of wood, construction in stone began in 1043 under Archbishop Bardo. The current Gothic structure was started around 1267 and completed around 1340, retaining the original floor plan with a double choir arrangement. Between 1462 and 1499, the cloister was added to the south side. Baroque furnishings were lost in 1857 due to the explosion of a powder tower, and the church suffered severe damage during World War II, particularly during the air raid on February 27, 1945.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:St_Stephan_zu_Mainz.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 49.9956, 8.2689