The remains of the Castle can be visited
Blankenstein Castle (Ruin) (Gladenbach)

Near the town of Gladenbach in the Marburg-Biedenkopf district, the Blankenstein Ruin, a former hilltop castle, sits at approximately 336 meters above sea level. Its strategic location once served to protect an important trade route.
History+
The construction of Blankenstein Castle is presumed to have occurred before 1237, intended to secure the “Cologne-Leipzig Road.” Originally owned by the Lords of Merenberg, disputes arose over the castle. In 1248, it was captured and destroyed by Landgravine Sophie of Hesse to secure the inheritance for her son Heinrich.
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: 50.7716, 8.5690