To be privately owned
Gnadental Abbey (Neuss)

The remains of the former Cistercian monastery of Gnadental, also known as vallis gratiae, are now located on the outskirts of the Neuss district of Gnadental, by the Erft river.
History+
The exact founding date of Gnadental Monastery is not recorded but can be indirectly inferred from old documents. As early as 1250, an indulgence was granted to visitors of the monastery church, which was reconfirmed in 1265. Two years later, in 1252, the monastery was placed under the supervision of the Cistercian monastery of Heisterbach. In 1474, Gnadental suffered severe damage from looting by Burgundian mercenaries but was rebuilt according to the Chronicon monasterii Campensis. During the Cologne War in 1585 and 1586, the monastery served as a base for troops of Elector Ernst. Alexander Farnese of Parma also used it as his headquarters during the siege of Neuss and celebrated his victory there on August 1, 1586.
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: 51.1726, 6.7152