Ivy Castle...

... or Chateau des Marais. If I recall correctly this is the oldest castle in Guernsey but there’s not much to see now. 
There is a geocache but it was on top of a wall and too high for me to reach.
The physical geocache whose co-ordinates I found from Candie Cemetery is in a lane not too far away. That one seems to be hidden in a patch of brambles and nettles so no luck there either.
But it was a very pleasant walk.