A dungeon is an isolated, mostly enclosed room/building/environment in which adventurers can find monsters and treasure. This definition is, in parts, too constrictive, too broad, or just plain wrong. Nevertheless, what we would call a "standard" dungeon would fit well into this definition, so that's what we'll use.
To the Gamemaster's general information pageThis page does not deal with why to create a dungeon. This should be halfway self-evident, as in the story taking place in a dungeon, or the exploration of the dungeon being the story. Instead, we'll talk about how to create one once you've chosen roughly what kind of dungeon it is (more about this later), where it is etc.
If this stuff is to theoretical for you, check out the example of a real (easy) dungeon setting!
There are two important questions in creating a dungeon (or similar environment):
The first question could be rephrased "form follows function". Instead of placing a series of caves, or a jumble of long, twisting corridors, or even a gigantic underground labyrinth with equidistant walls without thinking, it is important to make up an origin of the dungeon. (Important: it is perfectly acceptable to build any of the above if you have a good reason: "Well, I figure the old magician in that castle had gone mad, y' know. He appeared in town one day, and hired a lot of us guys, and had us build this gigantic underground lab'rinth thing with the equidistant walls. He was cackling all the time and later led some monsters in there, you should have seen them, gigantic things, well, and he said he had hid some enormous treasure in the middle, and we were supposed to tell all adventurers who came through here. Don't ask me about the plans, though. He made us forget them.")
The dungeon could be either natural, artificial or a natural area artificially rebuilt. In the case of a natural or previously natural dungeon, this will probably be because of water, or eathquakes, or other phenomena. You don't have to be a geologist here, just remember that water will seldom flow uphill, that nature seldom builds flat floors or right-angled walls, and other things (like the odd stalagmite etc.). Just build some caves or something, with a natural-looking opening to the outside (or none, if the adventurers are expected to dig to get in). Think about pools, stalagtites, holes for light and air (or leave these out, if you feel like it). Any artificial modifications are discussed in the next paragraph, since they would be "planned".
If it's an artificial dungeon, like an abandoned mine shaft, or an abandoned castle or tower, or an abandoned underground fortress (Or not so abandoned, up to you. Just remember that we're talking about the constructors here, i.e. the original inhabitants. Any current inhabitants other than the original ones are discussed later. If the underground fortress, for example, is still inhabited by those it's meant to house, fine.) then plan these dungeons as if you were the one who would use it. This means thinking about the entrance, about living quarters (if necessary), storage rooms, defense, etc. This might not apply to all dungeons (like mine shafts, or simple connecting tunnels), but it's worth a thought. Place waste disposals in underground fortresses, place kitchens and libraries in abandonned castles, place tool niches and torch holders in mine shafts, or whatever other sensible thing would go in here that the origninal builders would put here.
Next come the current inhabitants. OK, it would be perfectly sensible for dungeons to now have nothing living, moving, or threatening in them. That's not really the point of a dungeon, though. (Although you could fill a dungeon only with traps and obstacles). For the sake of encounters, though, we'll assume some current inhabitants. If they're the same as the original inhabitants, they probably already know how to live there. The others, on the other hand, would not. Think about how the "other inhabitants" (monsters, animals, undead etc.) would get into the dungeon. What led them here? What did they do when they got there? These questions are equally valid for "pre-infested" dungeons, i.e. those planned to contain monsters and other threats, for whatever reason.
Instead of just asking questions, we're going to try to give some possible answers. Of course, these are just a few possibilities. It's up to you to think of some more. Not all these questions need answering, of course. It is normally totally irrelevant where the inhabitants came from, or why. But it makes a more logical dungeon setting when the creatures do other things than wait for adventurers to prey on.
There are, for role playing games, two kinds of creatures to be found in dungeons. No, we don't mean threatening / non-threatening or intelligent / non-intelligent. We mean fixed encounters and random encounters. Fixed encounters are normally in one place, and occur once, when the adventurers meet them. Anything important to the story will probably be fixed. Random encounters normally take the form of random encounter charts. Every so often, or when the adventurers enter a room, or when they've been to loud etc. the Narrator checks whether an encounter occurs (oder bestimmt einfach, dass es so ist), and if one does, rolls several dice, adds them, and then checks on his / her pre-defined table to see what encounter appears. More about these charts later. The following paragraphs deal not only with creatures for fixed encounters but also give guidelines for selecting creatures for random encounters.
Probably the most important question for the inhabitants is their reason and method for being there. These can be of the following (though this list is by all means not exclusive):
Now that you know why and how these creatures exist, you can determine where they will roam, what they will normally be doing when encountered, and how they "influence" their surroundings. This might include separate encounter charts for different areas, things like lairs or nests, and "garbage heaps", like piles of bones etc. It is (in most cases) definitely a lot more fun and exiting to gradually see more and more signs of a carnivore, describing heaps of bones, a lair in a corner of a cave, the small of a large animal, signs of large paws, and finally confronting the mountain lion than to simply say "These's an ogre in the room and he attacks.", even though the ogre might be more powerful than the mountain lion. It's storytelling, however, that makes role playing games interesting!