The Fortress can be visited
FortressUlm
Federal Fortress-Fort Unterer Eselsberg

The Federal Fortress Ulm-Fort Unterer Eselsberg is part of what was once Europe's largest fortress system. Located in Ulm, Baden-Württemberg, at an elevation of approximately 549 meters above sea level, this fort served as a component of the defensive lines of the Federal Fortress Ulm.
History+
The Federal Fortress Ulm was constructed between 1842 and 1859. Fort Unterer Eselsberg, a part of this complex, was built under the direction of Faber du Faur from 1848 to 1856.
Source: Wikipedia · Licence: CC BY-SA 4.0
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: 48.4105, 9.9637