How bout some kind of trap...not sure where it would be set, possibly if the lock gets picked...but something that could alert some kind of 'guard/pet' that would come check out the room. If it finds someone, it will try to guard the room, if its a AM or thief and they are in the shadows and the guard doesnt see them, then they will move back to wherever they were summoned from.
Or, a trap that if the lock is tampered with unsucessfully, then all other doors in the castle automatically close and lock, effectively putting the castle on lockdown. There could even be some sort of 'spirit' voice that booms out 'Beware! Intruder walks among us'....or something like that.
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'I leased my soul to the Devil...with an option to buy'
