The remains of the Castle can be visited
Greene Castle

On a mountain spur above the Leine Valley near Kreiensen-Greene in Lower Saxony stands the Greene Castle Ruin, a former spur castle. Today, it is primarily known for its striking keep.
History+
Greene Castle was built in 1308 by the Nobles of Homburg to secure their possessions and an important trade route in the Leine Valley. After the death of the last Homburger in 1409, the castle passed by contract to Duke Bernhard of Brunswick, which, however, led to a conflict with the Bishop of Hildesheim. In 1414, the Welfs ceded the castle to the bishop for 12,000 Rhenish guilders, but reserved the right of repurchase. It was not until 1499 that the Welfs managed to raise the sum and take possession of Greene as an administrative district. In 1553, the castle was destroyed by Vollrad von Mansfeld on behalf of Margrave Albrecht of Brandenburg-Kulmbach, but was immediately rebuilt by Duke Heinrich the Younger. During the Thirty Years' War, the castle suffered further damage and fell into the hands of the Imperial forces after the Battle of Lutter.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: https://commons.wikimedia.org/wiki/File:Burg_Greene_von_Westen.jpg?uselang=de
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: 51.8542, 9.9344