The remains of the Castle can be visited
Krukenburg Castle

In Bad Karlshafen-Helmarshausen, in the Kassel district, Hesse, the Krukenburg castle ruin is situated at approximately 181 meters above sea level. It is particularly known for its unique cross-shaped rotunda church at the center of the complex.
History+
The history of Krukenburg is closely linked to the neighboring Helmarshausen Monastery. As early as 1107, a church building began to emerge in the middle of the later castle complex. This central building, whose floor plan follows a cross around a rotunda with a diameter of over 13 meters, was completed in 1126 and consecrated to John the Baptist.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Rainer Gindele
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: 51.6324, 9.4590