Christuskirche was built between 1896 and 1903 as a Protestant counterpoint to Mainz's Catholic cathedral. Its large Renaissance Revival dome makes it one of the easiest landmarks to pick out near the Rhine.
The church is worth a look for its scale and symmetry, and guided visits sometimes include the tower. Tower access can close in spring and summer to protect nesting peregrine falcons, so check locally before planning around the climb.