The castle is in Hastenbenck, not far from Hameln. Right next to it there is a small park or field. The place was closed then. I'm not sure if you can take part in some tours here.
Hastenbeck Castle is one of Edgar Wallace's film locations. The movie "The Uncanny Monk" was filmed here, in my opinion it is one of the best Edgar Wallace films. Unfortunately, the castle is privately owned and can only be viewed from afar.