Making a Believable Dungeon
One of the most iconic parts of any tabletop roleplay game is the dungeon-delving aspect; your party of friends traveling deep into ancient labyrinths, forgotten ruins, and underground caverns in the search for loot and adventure. While any location may be considered a “dungeon” in a broad sense, each location carries with it a different set of hazards and opposition to try and thwart any unwelcomed visitors. Making a “dungeon” believable in the context of whatever world it is taking place can be a bit tricky, so let's go over some of the things to look at while making a location for a party to venture through.
Location, Location, Location
The first aspect to look at is where your dungeon is located, as keeping this in mind will help you figure out a lot of other things later. This is not to be confused with the architecture or design of the dungeon, as even in our modern-day world we have places like the catacombs beneath Paris whose design is in stark contrast to the urban streets just above them. Don't let your location dictate the aesthetic of the environment, especially in a world full of magic or technology. Instead, let it narrow your focus on what kind of hazards or denizens may seek to block a group trying to make their way through the location.
Inhabitants
Sometimes, you may know exactly what kinds of creatures will inhabit your halls of horror. Other times, you may have a rough idea. Say you want a dangerous group of thieves living in the forgotten ruins beneath a city. They may be dangerous, but maybe not numerous enough to inhabit the entire structure, so instead they allow for the natural denizens of the deep to serve as deterrents and early warning systems while they use secret tunnels and passages to make their way safely through the place. What kinds of creatures would this organization deem to be dangerous to leave lurking about while still keeping a natural threat between them and their would-be pursuers? When choosing enemies to fill a dungeon, try to think about how the basic needs of those creatures would be met. How would they interact with other entities that may call the place home? Are they natural to this place, and if not, how did they get there?
Hazards
One of the most common mistakes happens when traps and puzzles begin to make their appearance. While every dungeon should have its fair share of mechanical or magical obstacles, the placing and nature of those obstacles should make sense for the environment they are found in. If you place an acid pit that is unpassable save for flight, then how did the creatures that lack the ability to fly later on in the dungeon manage to get there? If you have a hallway loaded with magical wards leading to the bandit's lair, you need to have a way for the bandits to traverse, or potentially bypass, their own security. A Behir that has made its home in the ancients dungeons of a ruined castle is not intelligent enough to bypass the multitude of spiked pits and falling blades that you had planned for your party without triggering them all itself. That isn’t to say that “because magic” can not be used in some instances, but relying too heavily on that reasoning can destroy the credibility of the world you are playing in. Make sure to have a good reason to use the “because magic” excuse. If the traps keep triggering and resetting, why would the Behir have made this place its home in the first place? If nothing else, use sprung traps to indicate to your party that something is or has at some point, already been here. This can add an element of drama to a dungeon.
Think Like a Designer
Take a look at any house, apartment, or place of business, and you can usually pick out the design layout. You can see how people are meant to flow through the space, and how each space leads to the next. A good design will hardly get noticed, while a bad design will almost immediately raise eyebrows. In fact, your attention is usually drawn to the inconsistencies in a layout pretty quickly, such as odd placements of hallways, bathrooms, or even windows and doors. Keep this in mind when designing a dungeon. Try to visualize how each room related to other spaces around it. Why put a storeroom full of ale on one side of the dungeon when the mead hall and kitchen are on the other side? Why is the bandit lords chamber right next to where the prisoners are kept? If a forge is located in the basement, where does the smoke escape to, and how is wood or fuel brought to keep the fire lit? While scientific or mystical means can accommodate some of these issues, just make sure to be consistent with their uses.
The Loot
This is the real reason why most groups try to take on the dangerous and unknown, and as such, is probably scrutinized the least. Most parties don't care HOW they get the treasure, just as long as they get it. The warrior is not going to care how that flaming battle-ax +2 made it into a gnoll den while none of the gnolls thought to actually use it. They just want the dang ax! Still, adding a bit of common sense to your rewards never hurt. If you plan on putting a chest full of gold someplace for the party to find, at least have an idea of how that chest ended up there, or why someone or something would have put it there in the first place. It would make little sense for bandits to have a storeroom full of magical weapons, yet they themselves are not using them. How did a powerful artifact make it into a cave full of dire wolves? If the item was so special, surely someone powerful would have come to claim it by now. Sometimes putting unanswered questions like these in a dungeon can build a lot of the world, and add adventure hooks later down the road.
Never Lose the Creativity
With all these limitations to keep in mind, it may sound like creating a realistic dungeon would be incredibly boring. The most important thing to remember is that you are playing a game. Let your creativity flow! While you may be limited to the world you are playing in, be it mystical, futuristic, or modern-day, that does not mean that you cannot be creative in your thinking and design. There are some amazing places in our real world that push the boundaries of what people viewed as possible, and they did it without magic or nano-technology. Don't be afraid to be fantastical with your designs and introduce your players to places they will never forget.