The Fortress can be visited
Museum
Grauerort Fortress

In Abbenfleth, near Stade in Lower Saxony, Grauerort Fortress is a 19th-century Prussian military architecture. Originally built to protect the Elbe River and the Port of Hamburg, this complex is now a museum.
History+
Between 1869 and 1879, the Prussians constructed Grauerort Fortress to secure the Elbe from enemy ships. Its strategically advantageous location on an elevated old marsh near the shipping channel was intended to provide rapid protection for the Port of Hamburg, especially during increasing tensions with France. The fortress was provisionally operational during the Franco-Prussian War of 1870/71 but was never involved in combat. Due to rapid advancements in weaponry, it lost its military status by 1895 and served as a mine depot from 1914, particularly after World War I. This use continued until the end of World War II. After the war, the British confiscated the undamaged fortress, which served as emergency accommodation until the late 1950s.
Image: https://commons.wikimedia.org/wiki/File:Grauerort_facade.jpg?uselang=de
Location: 53.6778, 9.4903