The remains of the fortress can be visited
Fortress Koblenz-Fort Rheineck

Fort Rheineck, a component of the Prussian Fortress Koblenz, is situated on the Nöllenkopf in today's Niederberg district. It evolved from a powder magazine into a fort and played a role in the defense system of Oberehrenbreitstein.
History+
The origins of Fort Rheineck date back to 1820, when a powder magazine was established. This was expanded until 1831. Renovations and expansions followed in the 1860s and 1870s, before the complex officially received the name Fort Rheineck in 1880. It was connected to Werk Pleitenberg via a link line, usable for both artillery and infantry. After World War I, the Treaty of Versailles mandated the defortification of the Koblenz installations. The demolition of Fort Rheineck began in March 1927 and was completed in August of the same year. Only the casemate wall and a hangar were spared from destruction.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Reste_des_Forts_Rheineck.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: 50.3727, 7.6158