The Fortress can be visited
Strasbourg Fortress

In the east of Strasbourg's old town, the Fortress Strasbourg-Citadel stands as an example of military architecture. It is now an accessible historical monument.
History+
Following the French occupation of Strasbourg in 1681, the Fortress Strasbourg-Citadel was constructed between 1682 and 1685. The plans for this pentagonal bastioned fortification originated from the military engineer Sébastien Le Prestre de Vauban. It served to secure the city and was a central component of the French defense strategy.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Parc_de_la_Citadelle-Strasbourg%283%29.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: 48.5759, 7.7746