Please don't read this post if you're playing in a game of mine right now
Group A has quickly decided that group B is the enemy. Last session, they discovered that group B had made a bridge over a pit trap at the entrance to the dungeon. They wrote "Thank you" on the wall - then replaced that bridge with their own, which is rigged to collapse when the other group walks over it. At the bottom of the pit, they put up spikes (d8 damage). On one of the spikes, they nailed a new character sheet.
Later, they set fire to a plague giant which rampaged through the town. Unbeknownst to them at the time, this was the town where Group B had ended their last session, asleep.
I think this might be the start of a war.
Everyone has a dungeon inside them
You might have noticed that the whole thing is drawn over a transparent map of Vornheim. This is the big deal about the campaign: The entire wilderness exists in the minds of the people in the city above. As the people in Vornheim sleep, their hopes, dreams and fears drift down to create this wilderness. When someone hides something, that secret twists into a dungeon down below. The monsters are all walking manifestations of the secret terrors and lusts of the people above.
What's the point of this psycho-symbolism?
This campaign was inspired by the West Marches, a wilderness campaign with the strict guideline: the adventure is in the wilderness, not the town. The idea is that the wilderness and town are opposing forces, and every moment you spend making one interesting will take away from the other. Being in love with town adventures, I wanted to try something different.
With this campaign, everything you do in the wilderness directly effects the town. Through the city, the players have influence over the wilderness: if you kill someone who was dreaming a monster, that monster will disappear. Through the wilderness, they have influence over the hearts and minds of the people in the city above. Killing a monster, setting fire to a forest, planting or building things - all the actions they take in the wilderness will change the people in the section of the city above them. As below, so above.
Here's a trick Group A has tried. Coffee imports are currently banned. They found someone who was dreaming a tower in the wilderness below, then paid her to chug as much coffee as possible. As a result, her tower has sprouted into a coffee plantation. They can harvest it, bring it up to the city above, and make a fortune off black market dream coffee.
Of course, then they got distracted, and the coffee was stolen by thieves, and their cleric got killed by the thief king, and now they're raiding a library to find out how to resurrect his soul into the body of a life-sized puppet. You know how D&D works.
This is the sheet I use for encounters. I drop a few dice on this piece of paper, in the rough location of the party: Whatever they land on is what's roaming around today. In theory, this means the party will mostly meet things nearby, with some chance of the dice rolling away and giving an encounter from far across the map. In practice, this piece of paper is a little too small , and the dice keep bouncing off the paper completely.
Treasure is obsession. When someone is obsessed with something, they turn it over and over in their mind, like a clam with a piece of dirt. The result is an image of what they care about most, encased in precious stones or metals and buried at the bottom of their mind. If you take that treasure, they will forget whatever they were obsessed with. If you put a new piece of treasure inside a dungeon...
Eventually, player characters will twist their own little dungeons or strongholds into the landscape. These will start as tiny little huts at level 3 or so, expanding out into giant fortresses and temples, custom-made around the things they worry about.
PC's may be hunted by their ideal self or their feared self. Villains they killed in the city above may still exist in the mind-wilderness below as terrifying caricatures of their real selves.
My players haven't figured the full implications of this stuff yet. I want to let them discover the rules of the place themselves, but I think I've dropped the ball a little. I haven't given clear consequences in town for the things they've done in the wilderness. This is a learning process, and I think I'll get better at it as I go on.