Well, one thing that comes to mind is that Castle Scarwall can repair itself - slow enough for it not to really matter from the PCs point-of-view, but at least the Chained Spirit itself should be aware that it can employ the castle itself against the invaders, and by the time the next group foolishly comes knocking a hundred years from now, everything will be fixed.
So after the player's initial foray into the castle, assuming they don't sweep the whole place out in the first sortie, would be that the Chained Spirit directs its corporeal undead (like the minotaur skeletons who should be strong enough to do this?) to weaken certain sections of the walls, floors, and ceiling to bring down on the invaders.
- Have a group of minotaur skeletons smash through a weakened section of wall while they're in the middle of fighting another group of undead
- Pit traps due to weak sections of the floor that the minotaurs hacked up, either causing characters to fall, or at least have to deal with the huge obvious hole in the floor blocking their progress - particular if they can collapse some of the stairs between levels. Your PCs should have access to things like Fly, Spider Climb, etc. so even a completely collapsed stairway should not be insurmountable at this point.
- During a fight with some of the minotaurs, they could attack the ceiling, bringing a weakened section down on top of themselves and any PCs nearby