The remains of the Castle can be visited
Old Castle Hardenberg

Hardenberg Castle Ruin, also known as Alte Burg, is in Velbert-Neviges. Today only a mound and an embankment remain above ground.
History+
Hardenberg Castle served as the ancestral seat of the Hardenberg family. Archaeological investigations in 1888, led by Gerhard August Fischer, uncovered a nearly square castle area with rounded corners, surrounded by a moat and protected by a curtain wall. Access was gained via a drawbridge to the east. A shield wall on the northeast side protected two main buildings with slate roofs, while a third was located in the western corner.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Gerhard August Fischer (1833-1906) (?) / Public domain via Wikimedia Commons
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: 51.3136, 7.0786