The remains of the Castle can be visited
Altenberg Castle (Ruin)

The Altenberg ruin, a former hilltop castle, sits on a steep, wooded hill near Füllinsdorf. Much of its history remains obscure.
History+
The exact construction period of Altenberg Castle is not documented, but archaeological findings suggest it was built in the 11th century. There are no written records of its builders or inhabitants. A possession of Schönthal Monastery in Füllinsdorf from 1226 suggests that the castle once belonged to noble land. When and why the castle fell into disrepair is also unknown. Extensive excavations were carried out between 1982 and 1987, during which the wall remains were uncovered, preserved, and the site was made accessible to the public.
Source: Wikipedia · Licence: CC BY-SA 4.0
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: 47.5096, 7.7397