The Castle is Alive

Old D&D dungeon rules sort of worked like this. Resident monsters could all see in the dark but surface people needed light sources or racial infravision to see in the dark depths. Doors opened for monsters automatically but required strength checks for surface invaders to force them open and they could swing shut behind you. The dungeon was subtly hostile to outside surface people coming into the underworld.

Shuffle the rooms

What kind of "living" are we talking about here? Walls made of flesh? Made out of dreams? Malevolent genius loci like the Overlook from The Shining? Linked to an entity like Dungeon Keeper/War For The Overworld? Demon posessed? Manifestation of a god like Malfeas from Exalted or Tartarus from Greek mythology?

jdrakeh said:

I forget which Fafhrd and the Gray Mouser collection it's in, but one of them contains a story about a living (and very murderous) castle.

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It's an old story, but that's still one big spoiler there, given that a lot of the story's impact relies on not being sure what's going on until fairly far in. Don't ruin it for those who haven't read it.

The story is Jewels In the Forest, which was published in Swords Against Death among other places. The building isn't really a castle proper, more a cluster of domes with an attached tower.

Obvious inspiration would be Castle Heterodyne in the Girl Genius comic, although it's probably more deadly (and possibly too mechanical) for what you're after. Still a fine example of what a fortress inhabited by generations of mad scientists might look like, though.

If nothing else you can probably mine some trap ideas from it.

The 4e D&D Monster Vault 2 (Threats to the Nentir Vale) has a Wandering Tower entry that could be helpful for this.

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*Damaged doors/walls/furniture heal
*Walls, floor, and ceiling contract to crush them
*Apparitions appropriate to the rooms and/or of former inhabitants à la The Shining
*Rooms that rearrange themselves
*The building casts spelks/psionics on them
*Absorbing people into the walls/floor
*Furniture animates like the bathtub in Ghostbusters
*Walls spawn golems or earth elementals
*Hearth spawns fire elementals
*Disorientation
*The bugs, rats, spiders, etc are possessed
*Traps spawn in
*Pervasive draining effect
*Doors and windows open and close on their own
*Doors disappear and reappear
*Fear aura
*Magical worlds inside paintings like in Mario 64
*Statues and d6ecorative suits of armor become animate
*Creatures that are more like spirits of the place than independent creatures (like the imps from Dungeon Keeper and War For The Overworld)
*Unnatural heat/chill
*Guards and Wards effect
*The building itself moves around
*Bigger on the inside than the outside
*Slow sanity loss
*Some sort of parasites, like building parasites
*Unnatural light/darkness
*Unnatural silence
*Bile pooled on the floor and/or dripping from the ceiling
*Unnatural emotional aura
*House has standard intelligent item powers
*Respiration; air moves in and out rhythmically
*Made of meat
*Made of living wood that's still growing leaves
*Full of nectar and/or stenches that attracts insects
*Full of sap that makes people get stuck
*Eyes in the walls
*Chandeliers and bells from bell towers and stuff like that fall from the ceiling onto people below
*Angry garden of animated plants
*New rooms suddenly spawn in
*Fire shoots out of fireplace/candles/torches etc.
*Exploding bottles in wine cellar that spray glass shards everywhere to damage intruders
*Damaged walls and doors spray blood
*Fantastical (and anachronistic) utility system, like blood circulating in pipes or an HVAC system that inhales and exhales
*Lit with those glowy things that grow out of the heads of female anglerfish
*Nails and floorboards pop themselves up to creat tripping hazards
*The individual stone blocks making up a wall unstack themselves and chase the PCs around
*Illusions
*Everything is moist, too moist to burn
*Mold and fungus and spores everywhere

I had a living castle I created for a Legend of the Five Rings scenario. It started out looking like a normal castle, but as the PCs spent more time in it there was evidence of something. The slight tremors they felt were the thing's stomach growling, they could feel air moving as the castle breathed in and out, the walls were like flesh at times, and it was slowly driving everyone mad.

Lots of good ideas in this thread, but too-much-of-a-good-thing definitely applies here. The first time that a painting hops off the wall to chase me around, I become a BIG art critic. Then if a rug tries to taste me later (or worse, swallow me), I'd get a real bad case of Don't Do Anything - It Might Kill You.