The Fortress can be visited
Fortress Koblenz-Fort Asterstein

Fort Asterstein, a historic fortress in Koblenz-Asterstein, Rhineland-Palatinate, is an integral part of the Prussian Fortress of Koblenz and the Pfaffendorfer Höhe system.
History+
The construction of Fort Asterstein took place between 1819 and 1826 as a main work of the Pfaffendorfer Höhe system, built in a similar style to the neighboring Ehrenbreitstein Fortress. In 1847, it was named in honor of the Prussian General Ernst Ludwig von Aster, who had rendered services to fortress construction in the Rhineland. As early as 1830, the fort was first occupied by troops. Throughout the 19th century, it secured the cities of Koblenz and Ehrenbreitstein along with other installations. A transformation occurred in 1865 when the northeastern rampart was extended and a second front was created. Until 1918, it served as a Prussian barracks, after which French occupation troops used the facility until 1929.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Fort_Asterstein_2007.jpg?uselang=de
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: 50.3549, 7.6155