The remains of the castle can be visited
Caldern Castle

The Caldern Castle Ruin, a spur castle whose exact period of construction and former owners remain shrouded in the mists of history, can be explored.
History+
The origins of Caldern Castle are not fully documented. It is believed that the complex was erected in the 11th century on the foundations of an older structure. A tragic event occurred on December 2, 1944, when a German fighter plane crashed into the mountain after being shot down by an American fighter plane. The young pilot lost his life; a memorial stone has commemorated this tragedy since 2007.
Location+
The Caldern Castle Ruin is situated at an elevation of approximately 308 meters above sea level in the Caldern district of the municipality of Lahntal, in the Marburg-Biedenkopf district of Hesse, Germany.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Burg_Caldern#mediaviewer/Datei:Burg_zu_Caldern.jpg
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: 50.8391, 8.6549