GM’s Guide: Building a Believable Dungeon

Building a Dungeon Crawl

There are three keys to building an effective dungeon crawl: Believability, variety, and non-linearity.


The first key is believability. Dungeons are sometimes laid out essentially (or even literally) at random, with the first room containing a pack of goblins, the second room containing an ogre, and the third a displacer moose. If the players attack the goblins, the ogre won’t hear and come to investigate. If they kill the goblins, retreat, rest up, and return, the first room will still be full of dead goblins and the ogre will still be waiting around in room two. The dungeon will have no notes for what the ogre and goblins and displacer moose are even doing there. They’re just the dungeon, a bunch of monsters camping on a bunch of treasure for players to kill. This problem is getting more and more rare as published modules and adventure paths are getting more and more popular, and they almost never make the mistake of having a dungeon whose inhabitants don’t even have any reason to be there, but in the interests of being thorough: Your dungeon should have a reason for existing. The monsters should live there for a reason. They should have treasure for a reason. Preferably they should pose some kind of actual threat to the surroundings rather than just being murderhobo targets.

Believable Layout

The reason why the monsters are in the area should inform the way they use the dungeon space. If they’re raiders, they’ll need a place to keep their loot and may not be as organized as if they were a forward attack base for an invading army. If they’re scouts for a dark lord, they’re probably much more interested in keeping exits open to flee and report back than if they’re a stronghold that’s meant to serve as a secure location from which to patrol the area and to resist siege.

The reason why the dungeon was originally constructed should also inform its layout (which we’ll discuss in more depth when talking about non-linearity). A mine that’s been overrun by kobolds will have a very different layout from a dungeon intentionally constructed as a fortress. It’s particularly important to bear in mind this aspect of believability when placing traps. If the dungeon was not originally meant to repel invaders, then unless the current inhabitants are themselves engineers and have access to enough manpower for serious remodeling, they probably can’t build any traps directly into the walls or floors, like a dart trap triggered by a pressure plate. However, they can still set up crossbows at one end of the hallway and rig them to go off if someone breaks a thin wire strung across the hallway at knee height.

Also, bear in mind that monsters have to actually live here. The path from the sleeping quarters to the toilet can’t have active traps going all the time or it’s going to kill more of the defenders than anything else. Traps between living areas and dining areas are eventually going to kill their owners unless they’re only armed when the dungeon is under attack. Traps which can be armed and disarmed can be just about anywhere, but bear in mind when running the dungeon that someone has to actually arm them. Whose job is it to do that, and how long does it take them? It might keep a goblin or two out of an encounter for a while.

Believable Monsters

On a related note, the monsters live in the dungeon. Even if they’re a raiding party or soldiers and have no civilian population (and most dungeons are, because most dungeons do not want to deal with the baby orcs problem, although if you do plan to use that particular moral conundrum in your game, everything about monsters living in a dungeon used as an army base goes double for a dungeon used as a village), soldiers and raiders still need to eat, sleep, and play a rousing game of “can the human eat it?” to keep up morale. They need places to eat, places to sleep, and if they’re going to be here longer than a week, they need places to relax, in addition to guardhouses and chokepoints and so on. A significant chunk of the dungeon’s population will be eating, sleeping, or relaxing until the alarm is sounded, and only at that point will they grab their weapons and get to battle stations. Particularly for monsters who were asleep and thus out of their armor, it may take 2-3 minutes from the sounding of the alarm to get ready for a fight and arrive at a guard station. Players may find monsters who, in an ideal (for the monsters) situation, would be fought together are instead split up in corridors and barracks pulling their boots on and heading to rendezvous points to form up with the other monsters in their cohort, and thus be able to pick off piecemeal an encounter that might’ve been much more difficult when fought all at once.

Sword fights make noise and monsters from the room just down the hall will probably come to investigate or reinforce their buddies. Disciplined monsters might have specific rendezvous points and might leave their friends to die in the room down the hall in order to head to that rendezvous point. Less disciplined monsters might rush straight for the fight. Cowardly monsters might stay in the room they’re in (unless bullied into doing otherwise by bigger and braver monsters), but they might also flee deeper into the dungeon and seek safety in numbers. Even if they do stay put, they’ll still bunker down and be prepared for a fight when the players enter, and probably have a pretty good idea of which entrance the players will come through and ready a bunch of actions to pincushion the first living thing that walks through.

If a dungeon is big enough (and many dungeons are), you might need a random encounter table to keep track of monsters who meander around, rather than keeping track of all the monsters yourself (something which is really only possible with a one page dungeon). Instead of figuring out exactly where each and every monster is, populate major rooms with specific encounters and then have reinforcements, patrols, monsters just wandering around restless and maybe a little drunk with their buddies, and so on be pulled from a random encounter table. Don’t make this table infinite, though. If players encounter five orcs led by an orog when they roll a 10 on the encounter chart, maybe have three of those random encounters (something which I usually write down as “five orcs led by an orog (3)”) and once they’ve encountered and slain or driven off all three of those encounters, that slot on the random encounter table is just empty. As the dungeon’s population grows more sparse, the odds that there will be anyone around to reinforce a guard room that sounds the alarm grow slimmer, the hallways are less populated and the party is less likely to encounter creatures at random while wandering them.

Another important reason to have a finite amount of random encounters is because, once alerted, dungeon lords will want to gather up those random encounters into a reserve force (monsters who are in static encounters hanging out in the barracks or currently asleep or what-have-you may also end up in a reserve force), which they will then drop on top of the party at first opportunity. Dungeon lords don’t have the omniscient perspective of a dungeon master, though. First a monster needs to survive long enough to sound an alarm. If the alarm doesn’t alert the entire dungeon (for example, if the dungeon doesn’t even have a proper alarm, just sentries who run to alert the dungeon lord when the entrance is attacked), then the dungeon lord will have to send out runners to alert all those random encounters and off-duty monsters to show up and form a reserve force.

Even if the alarm goes through the entire dungeon and all off-duty monsters immediately report to the reserve force, that’s still going to take much longer than the ~30 seconds (5 rounds) the party will take to finish up the dungeon’s first encounter and start exploring. This means that the party won’t be where they were when the alarm is sounded, which means the dungeon lord needs to find them, which means he’ll be chasing after alarms and scout reports trying to figure out where the party is going and beat them there, or at least catch them while they’re still fighting. If the reserve force ever does catch up with them, however, it’s going to be a lot of opposition dropping on the party all at once – probably enough for a party wipe if they aren’t smart enough to flee (at least far enough to find some terrain advantage like a choke point).

Of course, the dungeon lord needs to be ready for any kind of attack, including the possibility that the PCs are just a distraction from a lone infiltrator or another PC attack through another entrance. This means that they probably don’t want all the random encounters to join the reserve force. They need to leave some behind to check for infiltrators, and to sound the alarm if a second party attacks a different entrance and the reserve force needs to split in two. These tactics are rare for PCs, but mostly for metagame reasons that it’s more complicated to split the party and there’s almost never enough players in the group for an effective two-pronged attack. In-universe, the dungeon lord has no reason not to suspect that a nine-man adventurer party may have split into one party of four and one party of five and used one party as a distraction while the other makes a beeline for the treasury after the reserve force has mobilized in the wrong direction. What all that means is that the dungeon lord still wants his guard posts manned, even on the other side of the dungeon and still wants random encounters patrolling as much of the dungeon as possible. This is good, because it helps give the party a fighting chance against a fully alerted dungeon without harming believability at all.

What Do They Eat?

Monsters have goals. Usually this is some kind of military goal, they’re here to raid or invade and are using the dungeon as a base. The dungeon should be set up such that they can actually accomplish that goal. An army marches on its stomach, and it occupies a dungeon on its stomach, too. What’s the monsters’ food supply? Do they have several weeks or months worth of food reserves in their dungeon, and when those run out they’ll retreat back to friendlier territory? Do they feed themselves based on constant raiding, or by demanding tribute from nearby villages? Is there farmland surrounding the dungeon used to keep the dungeon supplied by monsters who flee to the dungeon’s safety when the adventurers roll into town? Do the monsters have Dwarf Fortress style underground mushroom farms within the dungeon itself?

What is the monsters’ goal, and how are they going about accomplishing it? If they’re here to invade, then they will need to leave the dungeon to do battle with defending forces. Do they plan to return to the dungeon and use it as their stronghold to occupy the area, or is it too remote for that purpose? If the former, how many guards do they leave behind when they leave the dungeon to give battle? If they’re using the dungeon to patrol and maintain control over territory, how much of their forces are deployed to those patrols? Do they come back each night (or each day, for nocturnal monsters), or do they patrol for days or weeks at a time? If they’re raiders, how much of their forces do they commit to a raid, and how long does it take them to get back from a raid target to the safety of the dungeon?

Make sure your dungeon’s inhabitants are actually accomplishing their goals (or at least trying to). This will almost always make them more vulnerable to attack, but that’s a trade-off that the dungeon’s inhabitants have to work with. On the subject of tradeoffs, a dungeon shouldn’t be prepared exclusively for an attack by a party of 3-6 powerful adventurers. They should be equally prepared for a lone assassin, a large army of weaker creatures, or a small number of powerful but distinctly non-human assailants, like dragons or beholders. Spreading guards out helps to detect a lone assassin, but also makes them more vulnerable to adventurers. If you’re focused only on defeating adventurer parties, that seems like a stupid move, but the dungeon doesn’t exist just for the players, and sometimes the dungeon lord might need to allocate their resources in a way that makes them more vulnerable to players in order to defend against other threats.

Can the dungeon defend itself against a large army? What about aerial opponents (if they’re a classic underground dungeon, the answer is “yes, because we force them to land in order to get into our dungeon at all,” but a military camp that works like a dungeon but is open to the air needs some kind of medieval AA)? What about one especially sneaky opponent? What about enemies with lots of breath weapons or other AoE attacks that punish tightly grouped opponents? What about enemies with high damage, single-target melee attacks that punish spread out opponents?

Lower level dungeons won’t have an answer for all of these threats. A goblin war camp’s response to an attack by an adult dragon is probably “grovel,” a bunch of kobolds relying on traps probably can’t reset them in the middle of a fight and probably can’t do anything against a large force of weak creatures (aka the Robilar strategy) except run, and a necromancer lord’s skeleton hordes might be dumb enough to fight tightly packed (all the skeletons just make a beeline for the nearest threat) even when it is advantageous to be spread out, so there is no point in having a strategy to funnel AoE enemies into a wide open room where a loose formation can be employed. The skeletons can’t actually adopt a loose formation, so why bother? However, monsters should still be prepared to deal with as many threats as their resources allow, even if that means they’re less specialized for defeating the player characters. The exception is if the player characters have proven to be far and away the greatest threat to the dungeon’s occupants, in which case they might reorient their defenses to deal with them specifically.

Believable Aftermath

For the sake of variety (which we’ll get into later) there are often multiple monster factions in a single dungeon, and they may be only loosely allied with one another, or even openly hostile to one another. If this is the case, after players retreat from a partially cleared dungeon, consider how this affects the factional politics. If a faction is weakened, another faction may push to exterminate them completely and claim whatever’s left of their treasure, as well as any valuable rooms of the dungeon (like the barracks with the good beds or a more defensible guardroom). This might result in a scramble for territory that depletes all the factions’ forces. Don’t spend too much time calculating exactly how many orcs die when mopping up remnant goblins and skirmishing with drow doing the same thing, but do reduce their forces a bit and have the orcs and drow go ahead and slice up the weakened goblins’ territory if none of the three really get along.

Even if all monsters in the dungeon get along, they still won’t just leave valuable territory vacant. While it’s possible that a wing of the dungeon wasn’t used for anything else except for the goblins to live in, and when some adventurers came through and killed all the goblins there was nothing left to do with that section, it’s also sometimes true that a section of the dungeon contains important parts of the living space like the kitchen or locations of strategic importance like a guard room or watchtower, and these locations are going to be repopulated by monsters taken from other sections of the dungeon.

All of this makes a dungeon more believable, the first and most important key to designing and running a good dungeon. By following these principles, the dungeon becomes a living thing with a purpose that reacts believably to player actions. This verisimilitude not only helps immersion, it also means the players can more effectively plan ahead. If the world makes no sense, then there’s no difference between a good plan and a bad plan. The more sense there is in your dungeons, the more it makes a difference how good the players’ plan is.

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