The well-preserved Scharzfeld castle ruins are located directly on the Karst hiking trail. An absolute must is to climb the remains of the upper wall, which I think is feasible for everyone. The view of the landscape is gigantic.
Directly at the castle ruins there is a small inn "Burgruine Scharzfeld", with a very nice landlady and also very passable food, I can recommend a stop.
Opening times: Thursday to Sunday 11.30 a.m. to 5.30 p.m., Monday to Wednesday is closed.
Here you can find the stamp box of the Harz hiking trail with the number 151.