The remains of the Castle can be visited
Kalsmunt Castle (Ruin) (Wetzlar)

The Kalsmunt ruin, a former imperial castle whose origins date back to the Carolingian era, stands on a basalt cone above the city of Wetzlar in the Lahn-Dill district of Hesse.
History+
The foundation of Kalsmunt Castle is attributed to Charlemagne around the year 785, who is said to have built it to control the already existing city of Wetzlar. Around 1180, Emperor Frederick Barbarossa expanded the imperial castle to secure the city, the imperial bailiwick, and the surrounding Wetterau region. Kalsmunt also served as an imperial mint, with a denarius minted under Barbarossa bearing the inscription "Calsmund" being known. In 1284, Tile Kolup, who claimed to be Emperor Frederick II, was imprisoned, interrogated, and burned at the stake at the castle in 1285.
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: 50.5493, 8.4956