The Fort can be visited
Fortress Mainz-Fort Josef

Fort Josef, also known as Josephsschanze, was a component of the second ring of fortifications in baroque Mainz. As part of the defensive structures of the electoral capital, it was intended to protect strategic interests along the Rhine and safeguard the city from enemy attacks.
History+
The Josephsschanze was constructed between May 1713 and 1730. The plans were drawn by the fortress architect Johann Maximilian von Welsch, acting on behalf of Elector Lothar Franz von Schönborn. This construction project was a direct response to advancements in weaponry technology, which increasingly rendered the existing fortifications of the first ring vulnerable. Fort Josef was one of five new forts designed to strengthen western defenses and relieve the older bastions.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Fort_Josef_Mainz_from_NW.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: 49.9955, 8.2561