Where is the rune stone on "castle courtyard" on Gauntlet Legends for PS1?

I'm on the castle levels on Gauntlet Legends and I keep missing a rune stone on "castle courtyard". I've re-done the level a few times now looking in ever corner and breaking the walls etc. but I still can't find it. I can't find any walkthroughs which tell me where it is either. Can anyone help?