It’s a moat, I’m not sure of the details.
De Haar Castle (Dutch: Kasteel de Haar) is located outside Utrecht, Netherlands. It is the largest castle in The Netherlands.
https://en.wikipedia.org/wiki/De_Haar_Castle
Fascinating history of Kasteel de Haar!
Lotta history in your link
Even includes how the Rothchild family offered up enough dowry for one of their daughters to marry into the dutch family & rebuilt the Kasteel!
Thanx a lot for the picture & background history