Living Space

We may divide the possible monsters at the dungeon's entrance into two general groups: (1) social monsters, including those with intelligence enough to be self-aware and directed towards a cause; and (2) low-intelligence creatures whose primary motivation is the obtaining of food, interrupted by periods of procreation.

Non-intelligent creatures, in particular, must be where food is; the dungeonscape is, if we accept the Earth model of caves, stone tunnels and the absence of things that grow in pitch darkness, bereft of nutrients — except for very tiny creatures.  If we're dealing with the second type of monsters above, we might view the outer dungeon as a "naturalised" setting, a place where creatures from outside occasionally come in for protection from the elements or as a place to breed, or where they might potentially hibernate.  Depending on the species, bats retreat to a cave during the cycle they do not hunt.  These things would apply most to creatures of animal intelligence, especially mammals, though in very hot climates there are reptiles and amphibians that seek caves for the coolness or the presence of water.  Non-intelligent animals, like insects, see a cave entrance as a place to lay in wait, as larger creatures move past the entrance; this would be the best time for a giant monster spider to strike the party, just as they're approaching within 10 feet of the open entrance.  This is how many hunters obtain their food; they employ camouflage, lay waiting and not wasting energy, until a large enough creature moves within range of a quick dash forward and SNAP!  Dinner.

This tactic is so effective that guerilla soldiers use it — and obviously that could apply to an orc or kobald laying in such a hole for days before relieved.  But set that aside for the moment.

Other forms of non-intelligent creature, like undead, attach themselves to a place because its where they died, or were raised in an unholy fashion and then abandoned.  They have no understanding of place or what they're doing, but if something or someone approaches, they automatically attempt to kill it.  This is done without any sort of strategy or logic, because as I say, "non"-intelligent.  Still, we should have some conceived reason for why these things are here at this time, and in this place.  They may be purposefully intended to die; their death may trigger some magical device that tells an intelligent monster deep in the dungeon that the outer perimeter has been breached, because the zombies are dead.

I don't want this to devolve into example after example.  The sense is that if the dungeon is occupied by creatures of lower intelligence, then they're just doing what's natural to that species or type.  They're either retreating into the cave for protection, or using it as a means to obtain food, or acting mindlessly as some monsters do.  This may not seem like much logic for a dungeon, but then this may be only the outer cave of some elaborate, deep complex.

Taking in the first option above, the intelligent creature.  If a humanoid lair is only a few hundred feet from the outer entrance, it makes sense to fortify the entrance and post guards; it makes sense for some of those guards to participate in patrols, to see if anyone is roaming around outside or just showing up to randomly homestead too close — having no idea there are 40 orcs living and working in this cave just a quarter mile away.  If the party should stumble upon such a lair, they'd have a fight right off; they might find themselves facing just four outer guards at first, but then moments later a second team, having spotted the party hours ago, appears behind the party to trap them at the entrance.  Now the party cannot leave or easily get inside, while fighting on two facings.  It's a tough way to start a dungeon.

In either case, if the cave is infested with bats or ticks or spiders, or if it's a humanoid lair, this may only be the first upper part.  The humanoids may not even be aware that there are other beings living deeper down; the party may not learn it until following some "abandoned" tunnel to it's end, detecting a hollow sound from the rock and breaking through to an entirely different dungeon below.  By laying out the dungeon in pockets, players can experience different sorts of monsters logically divided from one another by long tunnels, sumps that must be swum through to reach the other side, very deep holes that must be descended with ropes and other climbing gear ... whatever we can dream up.  The separation of monster lairs by geologic formations produces a different wilderness that, like the outside world, still separates various entities logically.  The silliness of many modules can be corrected simply by putting greater distances, and more elaborate obstacles, between one part and another.

Thus, the players may start out by killing the humanoids, but then may find themselves on an seemingly purposeless quest roaming around in a dozen miles of caves fighting non-intelligent serpents, a giant worm, a termite-like colony with two many warriors, before discovering an unlikely race of "deep gnomes" who are in a continous war with the race that lives in warrens even further down, separated by half a mile of clefts, extrusions, torrential streams and mud bogs.  If the characters are searching for some McGuffin they must have, such an arrangement would demand tireless persistence ... especially as no one they've met or fought has even heard of the beholder they're looking for.

Intelligent races set out defenses; they organise themselves into rational arrangements that allow the obtaining of arms and the protection of the offspring.  They need some sort of food to eat, so we must imagine a host of fabricated fictional non-chlorophyll based plants that can be grown, or passive slugs or warm-blooded beasts that can be raised as food.  They must have some way to occupy themselves, either in tunnel-building, farming, gathering and purifying water, making art, making tools and arms, telling stories to one another, eating and whatever.  Fire is impractical because there's no wood; and if there is a substance that burns, smoke would gather and ultimately would reveal their location, even if some sort of chimney could be sustained.  Yet how do you make hard metal objects without fire, and how do you make tools without either wood or stone?  The very fact of underground creatures defies any sort of logic — so invention must create circumstances in which these things may be feasibly pursued.  Perhaps there are plants that can be grown upon a ceiling, that consume smoke and produce food as a consequence.  The game Oxygen not Included has creatures that automatically expel coal, slime, light, crude oil, gasses and so on.  Any of these might live in our D&D underground.

Keep in mind that a dungeon may have multiple exits; some might be naturalised, some not.  Those that are not actively defended will accumulate creatures from the wilderness, or from places deep within the caves.  Those actively defended will have characteristics that makes them good places to dwell ... natural water sources, protective outcroppings, large natural spaces that reflect light and so on.  Creatures do not settle randomly.  They find a place that suits them and then they make adapy that space to their needs.  Just as we do.

No one digs a 75 foot tunnel if no tunnel is needed at all.  It's incredibly hard to dig through rock; the rock must all be deposited somewhere, which is more work and produces a huge dump of obviously mined rock that can't be concealed easily.  It's 75 feet we have to walk and walk again just to get from A to B.  Just as we build homes as tightly as possible to make them convenient for use, monster lairs would also be built so that you don't spend all day walking down ludicrously long hallways to get a bite or let someone know they're needed elsewhere.  Closely-packed rooms benefit communication, distribution of labour, protection against outsiders and communal feeling.  None of the dungeon layouts I've ever seen of a monster lair makes any sense on this point.

I'll put this down.  As the players in our example are about to fight a bunch of goblins, we'll need to talk about combat.