The Fortress can be visited
Prison
Fortress Ziegenhain

Ziegenhain Fortress, a water fortress in the Schwalm-Eder district, Hesse, is an example of Baroque fortress architecture in Northern Hesse. It was the fourth-largest fortification in Hesse during the Thirty Years' War and gained fame for its impregnability, leading to the proverb “As strong as Ziegenhain.”
History+
The origins of the complex date back to around 950 with the construction of a keep. In 1144, Count Gottfried von Reichenbach-Wegebach built a new castle between the old keep and an existing circular castle, which he made his residence and after which his family was named. Werner II. von Falkenberg added “The Stone House” in 1363. After the extinction of the Counts of Ziegenhain in 1450, the county fell to Landgrave Ludwig I of Hesse, who converted the castle into a hunting lodge around 1470. Under Landgrave Philipp the Magnanimous, the complex was expanded into a strategic water fortress from 1537 to 1546.
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: 50.9116, 9.2350