The Fort can be visited
Fortress Mainz-Fort Malakoff

The Fortress Mainz-Fort Malakoff, an example of Prussian military architecture, is an essential component of the former Fortress Mainz. Situated on the left bank of the Rhine, it shows the city's strategic importance in the 19th century.
History+
The construction of the Caponniere Malakoff, also known as Fort Malakoff, began in 1843 as part of the Rhine regularization. As a component of the left Rhine bank fortifications, this multi-story artillery work was expanded in several phases until 1873, with the main construction period between 1841 and 1848. The fort received its current name in 1855, inspired by the Crimean War.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Fort_Malakoff_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: 49.9968, 8.2817