The Fort can be visited
Fortress Mainz-Fort Weisenau

Fort Weisenau was a component of the Mainz Fortress and served as a bulwark protecting the city of Mainz in the southern area, adjacent to the Rhine. Remnants of this complex can still be found on the edge of Mainz Volkspark.
History+
The origins of the fort date back to the period after the Siege of Mainz (1793), when Austrian troops constructed an outer enceinte with lunettes for the Weisenau redoubt between 1793 and 1797 to secure a weak point. After the Treaty of Campo Formio, Mainz fell to France, and although the fortress was reinforced by French builders, they neglected the southern land section in favor of the Rhine front to the north. With the handover of the Mainz Fortress to the German Confederation in 1825, expansion began. Fort Weisenau itself was built from 1826 to 1831 by Austrian pioneers according to plans by Franz Scholl under the direction of Edler. It stretched over 250 meters and included three two-story redoubts. The costs were covered by reparations from the Second Treaty of Paris.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Fort_Weisenau4.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: 49.9877, 8.2938