One of the weirdest things about old school Dungeons and Dragons is how doors work.
From Book III: The Underworld & Wilderness Adventures:
Generally, doors will not open by turning the handle or by a push. Doors must be forced open by strength, a roll of a 1 or 2 indicating the door opens, although smaller and lighter characters may be required to roll a 1 to open doors. There can be up to three characters attempting to force open a door, but this will disallow them rapid reaction to anything awaiting them on the other side. Most doors will automatically close, despite the difficulty in opening them. Doors will automatically open for monsters, unless they are held shut against them by characters. Doors can be wedged open by means of spikes, but there is a one-third chance (die 5–6) that the spike will slip and the door will shut.
From this we learn 3 things:
Doors in the dungeon are usually stuck, requiring a die roll to force them open.
Doors automatically close behind the characters.
Doors automatically open for monsters.
Point 3 is especially strange and many have debated why this is. Consensus seems to be it’s just part of the “mythic underworld” in the same way that monsters can see in the dark but characters cannot, and a monster that joins the party instantly loses this ability. The rules are just different down here.
But leaving aside the “why”, let’s look at how these oddities affect game play.
Stuck Doors
First, stuck doors create barriers within the maze, reducing the number of available paths or discouraging certain paths. This makes discovering secret doors, alternate paths, and mapping the layout of the dungeon valuable, especially when the party must evade a powerful monster.
This rule also interacts interestingly with the rules for listening and surprise. Characters may attempt to hear noise behind doors to avoid being surprised. But characters have to roll a die to force open a door, and failing to open it will automatically alert monsters on the other side, eliminating the chance for surprise. This presents a choice between attempting to open doors that sound quiet (presumably unoccupied) or trying to force doors to occupied areas in hope of earning surprise.
Up to three characters can attempt to force a door, but will then be unable to immediately react. The choice here is between increasing the likelihood of opening the door, or having more characters ready to face what is on the other side. Worst of all, if the characters fail to hear monsters on the other side (such as silent undead) and fail to open the door on the first try, now the monsters are alerted and have a chance to surprise the characters.
Doors Automatically Close
Second, doors automatically close behind the characters, presumably becoming stuck again. This cuts off avenues of escape if the party finds themselves fleeing a powerful monster, and creates tension as the party desperately tries to force the door while fending off the monster. The choice for each character is clear: help try to force the door, or fight? This conundrum is extra delightful when the tanky characters have the best chance of opening the door. Again mapping the dungeon layout accurately, locating secret doors, and having many known alternate paths becomes extremely valuable. In addition we have two other strategies for dealing with closed doors: spikes and the Knock spell.
Once a door is open, characters may attempt to keep it open by wedging it with spikes, but there is a chance the wedge will fail and it will close again anyway. Is it worth the time and noise to wedge it open? May we need to quickly flee back this way?
Alternatively, the only guaranteed way to open a door is the Magic User spell Knock. At first blush Knock doesn’t have the combat utility of Sleep or Charm Person, but when a stuck, locked, or held door stands between the party and escape from a particularly nasty foe, its value in this game becomes immediately apparent.
Doors Automatically Open for Monsters
Finally, the strangest thing is that doors do not hinder the monsters - they open automatically unless held shut by the characters. The rules for physically holding a door closed against monsters are left up to the referee but should the party wish to continue fleeing and leave barriers behind them, one option is the Magic User spell Hold Portal. This spell keeps a door, gate, etc. magically held closed for 2d6 turns.
Hold Portal also lacks some of the combat utility of other spells but should the party become trapped or require a turn or two of rest (another oft-overlooked rule of dungeon exploration), they will be grateful indeed that the Magic User memorized this spell.
Knock and Hold Portal as Spells
As an aside, while Knock clearly has utility in locating secret doors and forcing open locked doors or gates, I believe its value is drastically increased and the need for a Hold Portal spell is altogether borne out of dungeon doors behaving in this very strange fashion. I’ve yet to see Magic Users memorize either in my own games but I imagine more stuck doors of this kind in my dungeons might quickly change that.
Conclusion
Though odd, the implications of stuck doors on game play are significant if wholly embraced. They create barriers that offer the exploring party interesting choices, enhance and reward good mapping as a mini-game, create tension as the party flees dangerous monsters for whom they are no obstacle, and birth two spells to tempt the Magic User that would otherwise be of suspect utility.