The remains of the castle can be visited
Unterburg Castle

The Unterburg castle ruin, also known as Zwingenberg Water Castle, is a historic site in Zwingenberg, Hesse. Although only foundation remnants are visible today, they are among the oldest structures in the city and were part of the city's fortifications.
History+
Presumably built by the Counts of Katzenelnbogen, the castle was expanded around 1250. In 1301, it likely suffered destruction during conflicts over customs duties, when the Counts of Katzenelnbogen sided with the Rhenish electors against King Albrecht I. Following this destruction, Count Wilhelm of Katzenelnbogen waged a feud against the Archbishopric of Mainz because reconstruction aid was insufficient. A settlement in 1312 led to the castle being transferred to the Archbishopric of Mainz, but the Katzenelnbogens were enfeoffed with it and received financial resources for reconstruction.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=984
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: 49.7218, 8.6131