There was a discussion on the osr subreddit the other day about players resting in the dungeon, and it occurred to me that if you iron spike or bar a door shut so you can rest but roll an ooze on the wandering monster table while resting, it might just be able to get through under/around the door and disrupt your sleep.
Thinking about whether there were other types of monsters that could do something similar, incorporeal undead and swarms also spring to mind. Maybe vampires if you let them go gaseous voluntarily. Any of your burrowing enemies like ankhegs, grey worms, purple worms, thoqqas, and xorns, though those will mostly be loud and noticeable.
There's an interesting parallel with Deep Rock Galactic here, where they introduced several enemies (the bulk detonator and the oppressor) explicitly to counter the strategy of building bunkers. These enemies can "force the door" by digging and are either difficult to damage from the front or explode massively. It kinda makes me wonder whether some of these classic D&D monsters that might plausibly pass through doors were originally developed as counters for resting in dungeons.
I think this has interesting implications for how you build random encounter tables for dungeon levels. If there's a "bunker buster" monster on your table, then the party is at risk of having their sleep disrupted if they barricade-and-rest on that level. I went looking for AD&D 1e's tables but still haven't found them. In OD&D Book 3, there's only the Ochre Jelly on the 3rd level table (though there sure are a lot of MUs who might be able to knock in a door). Moldvay / OSE is much more interesting through this lens, with the following levels container the following potential bunker-buster monsters:
- Nothing really (green slime isn't motile so doesn't count)
- Grey Ooze. Pixies hiding in the room you're bunkering in to prank you in your sleep would be pretty funny but I'm not sure it really counts.
- Ochre Jelly and Shadows (Gelatinous Cube too but it probably can't fit under doors?), Basic Adventurers potentially
- Grey Ooze, Ochre Jelly, and Wraiths. Rust monsters too, depending on how exactly you do your spiking. Expert Adventurers also possibly.
- Black Pudding, Ochre Jelly, Spectres, Expert Adventurers, possibly White Dragon
- Black Pudding, Purple Worm, Vampire, Expert Adventurers, possibly Red Dragon.
By the end there a solid quarter of possible wandering monsters are possible bunker-busters. But holing up in the top levels of the dungeon is probably pretty safe!
If players quietly rest 8 hours in the dungeon, they'll get 3 wandering monster rolls per hour, or 24 total rolls. Since the probability of a monster on each roll is 1 in 6, you expect four wandering monsters. So if a quarter of your table for a given level is bunker-busters, then resting will usually fail.
ACKS has heuristics for building dungeon wandering monster tables, with one-third beastmen, one-third mindless/animals, and one-third "men and monsters". It would be interesting to add a heuristic around bunker-busting enemies as well. If you have one on a 1d12 table, then it won't come up in 70% of rests (assuming 4 wandering monsters are rolled; there's a long tail of bad luck in the wandering monster checks). Two out of 12 means something gets through the door in 52% of rests. At three out of 12, the door only holds in 32% of rests. I think I kinda like 2 in 12 - you can do one ooze and one incorporeal undead, swarm, or burrower, so there's some variety, and rests are pretty close to a fair 50/50 shot. Or if you count Rival Adventuring Party, then the interruption isn't necessarily unfriendly, but it might still be an interruption.