I think most (fantasy) gamers start by going into “dungeons”. Dungeons are actually easier on the GM. He knows everything, and the players are contained to a limited space. Pretty quickly, things move to a “wilderness adventure”. These seem more realistic, because we all know that nobody actually builds dungeons. But with the added freedom comes a lot more difficulty for the GM. Now there is terrain, and nothing to hold the players into a restricted area.
Having disrespected dungeons, I think we all remember them fondly. That’s why I ran “The Killer Dungeon” many years ago. I gathered all my best traps and encounters, put them into one dungeon, and held a tournament. (The prize was a tiger’s eye topaz - we thought it was pretty cool!) Immediately after running that Killer Dungeon tourney, I started writing the next one. I never got to run it, but here’s how it went. I still think this is the most deadly dungeon setting in the world.
The main concept is that this is a base of rebel halflings. They are living in underground tunnels based in many ways on the VC tunnels. Nothing was going to be included that could not directly be explained by the people in the base itself.
Let’s start with the tunnels or hallways first. Assume that the halflings are 3’ tall. The tunnels are triangular shaped (flat floors, peaked ceilings). From an engineering sense, this is supposed to be one of the strongest ways to build a tunnel. So the peak will be 3’8”. This allows a 3’ halfling to run down the middle of the tunnel without having to watch his head. It forces human sized people to crawl on all fours - you likely couldn’t even crouch. The tunnels are only 3-4’ wide at the base. This is intended to leave enough room for halfling shoulders, but prevent anyone from swinging a slashing weapon. No magic swords, no axes - should severely cut down on most any adventuring party. The halflings inhabiting the base rely on crossbows and spears, with knives as backup weapons. Therefore they should be able to fight in the confined spaces. Remember - the big invaders are on their hands and knees; they may not be able to use their hands for combat or possibly even magic in the hallways.
To protect the hallways, there are guard posts. Basically, they widen the hall to 10’, giving the guards the ability to hide behind the tunnel walls and shoot their crossbows. I also like the idea of a defender using a pole arm to fend or basically block the hallway. Should someone try to force past him, he can use the pole arm to stab and slash, and the invader would have a very difficult time trying to get past. Sure, he likely would have the strength to rip the pole arm from the halfling - IF he had the room to maneuver and was standing on two feet. In a confined space, he’s a sitting duck for the spear or blade.
The hallways are also trapped. We’ll get into that later. But assume that the invaders come down the tunnels, manage to storm a guard post, and the halflings go off running. First off, halflings should be slower than humans in a sprint, but the halflings are sprinting and the humans are bear crawling. That evens the odds considerably. As the halflings run down the halls, they avoid the traps, because they know where they are. Put enough corners into the design so that invaders learn quickly that blundering around a corner often means a crossbow in the face. This also prevents the invaders from seeing the defenders avoid the traps. Meanwhile the guys retreating are dropping caltrops or even broken glass. While the broken glass only helps against people in cloth or leather style armor, the caltrops will poke through chain mail - like the chain mail glove. This isn’t only a distraction, but a possibility of messing up a warrior’s attack hand.
Meanwhile, the halfings have stout doors and the previously mentioned twists and turns. They can hide their lights (or rely on that mystic heat vision that every non-human has in some games). This allows them to listen and watch for the invaders’ lights. Typically the invaders’ lights will give them away long before the defenders give away their position.
OK - This is already too long, so we’ll end here and finish up next time.