This is Rabbit's Castle... it sits in the middle of a river gorge.  Upstream, about a mile or so, a waterfall stretches across the gorge.  Beyond the falls, a crater lake extends to the distant mountains.

The only access to the castle is on the near side - through a series of secret tunnels which lead to an opening in the side of the gorge, and the bridge... 

This is just a 1st ruff concept... the castle needs to be more exotic-organic looking, and probably larger - but still plausible, this is science-fiction after   all, not fantasy.