Depth-Based Adventure Sites
Many of us know the joy of describing a complex dungeon and watching our friends studiously attempt to chart it out on graph paper, in many cases distorted by the "picture-telephone effect." And mapping out a dungeon can be serious fun for a DM. But sometimes, adventurers want to explore a place which should, by rights, be so tremendously complicated as to be un-fun to map out. Consider the maze of sewers under a major city. They're a perfect dungeon in theory, but in practice they're literally miles of sprawling tunnels that pass over and under each other vertically. The same goes for catacombs, labyrinth dimensions, non-Euclidean Lovecraft ruins, haunted forests, and psychic purgatories (made of halls full of doors with, presumably, live-action train footage on the other side). All of these places are too vast and too maddeningly complicated for a team of adventurers to explore them in their entirety, and their aesthetic appeal largely comes from the fact that they're too densely complicated for a DM to create.
So, instead, whenever my players want to journey into one of these places, I sit down and prepare a "Depth Chart" which works much like the random dungeon generation tables you'll find in the DMG. But they're distinct in several ways:
A Depth Chart is designed for the maze-like adventure site, so it randomly generates more appropriate spaces and has built-in encounters unique to the site itself.
A Depth Chart assumes that the adventure site follows a gradient: it might be very mundane when wandering around near the entrance but get weirder, more dangerous, or more rewarding the further in you go.
A Depth Chart follows a more consistent logic and therefore PCs can learn to manage the danger level by responding to the cues.
Once the PCs learn the signs of their progression, a Depth Chart gives adventuring in a site a sense of rising tension.
A Depth Chart allows PCs to eventually (and inevitably, if they look long enough) find a specific goal in the twisted bowels of the site it represents.
Example: Sewers of Marion Hill
My most recent depth chart from my home campaign, when my PCs explored the sewers beneath Marion Hill, looks like this:
Because this is a sewer, the chart assumes that, between all points of potential interest are long channels or passages. Essentially, the DM rolls on table X between each decision the players make to determine what they come upon next, unless the players are backtracking. Rarely they'll encounter a specific "zone," a rare sort of site. These could be an interesting but unusual kind of location in the adventure, a dead end, or the entrance to a mapped dungeon or secret settlement or even a different sprawling complex with a different depth chart.
Now, you may have noticed that these four tables are meant to use a d6, but that there are more than six entries on three of them. That's where the "depth" part of "depth chart" comes in. We track a value called Depth which estimates how far from the "safe" entrance the PCs have ventured. When we roll on table O, M or Z, we add their Depth to the result.
So when the PCs first enter the sewers, they're only going to encounter things that fall between 1 and 6 on the tables for a little while. None of the scarier monsters are this close to the surface, and there aren't weird threats like Witch Eggs, and none of the zones are secret workshops or other sites that would be hard to stumble upon. On table M, you'll notice that a result of 1 yields some urchins-- they wouldn't be found too far from the surface, and once the PCs have accumulated even one point of Depth, it becomes impossible to roll the urchin result.
So how do the PCs accumulate Depth? That depends on the logic of the adventure site. In Marion Hill's sewers, and in a lot of places, the simplest answer is "going down." I made downward transitions abrupt and a little nerve-wracking: they are 20' ledges with ladders. Difficult to navigate in a hurry, hard to tell what's up ahead. These were the subconscious cues I gave them that indicated that they were getting into the parts of the sewers best left forgotten.
As you can see on the chart, there are other ways they could manipulate their "depth." Passing through a locked portcullis meant going someplace you weren't supposed to be. Getting directions that lead deeper in from sewer maps or dead men could do it. I suppose that if the druid had decided to ask rats for help, that might have worked, too, even though it's not on the chart.
And, of course, correctly interpreting and acting on clues will get you deeper into the sewers.
"Clue" here is intentionally vague. My players were looking for a man who had been laying "lectric cables" in the sewers, and so often they could follow the cables for a time. Then they'd go right through a bricked-over or recently collapsed tunnel and they'd wander around trying to find where it came out. For them, often a "clue" result on the tables meant they found the cable again, and it lead them deeper into the dungeon (and closer to their goal, which they'd find on a 10 on the monster chart or a 9 on the zone chart, so the more depth they accumulated the more likely they were to stumble across it).
But a clue can be any sign about what's deeper in the sewer and which way to go to get there. When the players were sufficiently deep and encountered a 4-way intersection with two clues, one was their cable and the other was a carved skull nailed over an exit-- a warning about the kinds of monsters at the depth they'd be entering.
Building A Depth Chart
I used four tables for the chart above, and I think it's the happy medium. More different tables and you slow down play while you roll all over the place and manage your folder. Fewer tables and you lose the kinds of emergent, dynamic situations that come from combining a situation with a threat.
You can certainly make your tables longer than I did for a much slower trek through more various creatures and sites, or use a different die-- I used a d6 because increments of 25% and 10% felt too "even" and predictable to me, and larger dice increase the range of depths that encounters stick around for. But the d6 is comfortable, and you probably have a bunch in different colors for ease of use.
You can even apply depth to the intersections table if you want the architecture to shift as the players trek deeper into the site.
Some important tips about Depth Charts:
Use dead ends sparingly. Obstacles can be common, and should be, but true dead ends should be rare. Since we're only using depth charts for these sprawling, endless mazes, you want the complex to feel too large to fully explore. The easiest way to do this is to have the PCs to see many more corridors than they wind up exploring. If you're worried about the space not making geometrical space, avoid describing lengths of corridors in concrete lengths (a short walk, a long walk, rather than 60' or whatever) and make sure to mention, from time to time, the slight sloping of uneven passageways.
The players should only very rarely come to a fork where there is nothing to inform their decisions. You don't want them guessing randomly, since sometimes the environment really is pointing them to their objective. Since there is no objective "right way" to go to reach their target, if they even have one, you have to make their decisions matter another way: by giving them control over what kinds of encounters they're having and whether they're getting out-of-depth, if they're clever enough to read the environment.
Instead, put a meaningful choice on every intersection result. A choice about what they'll be doing as they proceed. That's why, in my example charts, there's always a choice implicit in table X rolls. 1-- there's a monster in this passage; do you sneak by it or fight it or backtrack? 2-- Are you going to take a long detour or bypass an obstacle to keep going the same way? 3-- Fight a monster or bypass an obstacle? It's possible to encounter a 4-way intersection where all four paths are clear and indistinct, but it's only about a 0.61% chance. Even if encounters with monsters and NPCs are rare, find things to inform players about.
Stray Outside The Party's CR. The early-depth results on the monster tables (or obstacle tables, if you're putting in traps) can be easy encounters. Especially since our sewers were right under a highly populated city, it doesn't make sense for there to be noticeable threats right under the populace's noses. These areas should be pretty safe.
But don't be afraid to put some truly horrible monsters on the table, especially if they're not the kind that would take intelligent pains to hunt down the PCs if they try to get away. The "game" of these charts, for the players, is figuring out how deep they can get before they should really be fleeing back to the surface. There should be consequences for getting in way over their heads.
My players descended a ladder and encountered a shambling mound they had little hope of defeating, and were forced to flee. It was the "approaching monster" option at a 4-way intersection, and one of the other options was covered in witch eggs. Not wanting to be poisoned or for someone to get pounded into goo while they all climbed the ladder one by one, they ran down an unexplored hallway... to discover another sudden drop. Swearing, they ducked even deeper into the sewers to escape, knowing that things were only going downhill from there. True to form, the morbid clues and monster spool they discovered in the next intersection made them desperate to sneak back past the mound and escape.
Make Depth Transitions Dramatic. Make it slightly difficult to return to "shallower" areas, either by putting in a narrow ladder as I did or making characters squeeze through cracks or descend pits with rope. You could even just post a sign. "TRESPASS AT OWN RISK." Ultimately you want them to feel like they're crossing a line in the sand... because they are. You want them to feel like they might be "getting in too deep." They have to cross the threshold and take ownership of the inherent risks.
Of course, depth transitions don't have to be literal descent, and Depth doesn't have to be altitude so much as "distance from the entrance and from the safe, everyday world."
The more dramatic the transitions feel, the more "gameable" they become. You can even change the decor as Depth increases, if your players don't understand "subtle." They'll understand that the tunnels with bones littering the floor are more dangerous than the ones covered in mad scrawlings.
Lost in the Labyrinth
If you decide to build a depth chart, just remember that they're for simulating the feeling of being in a truly convoluted maze, when the players have to wander hopelessly through the site to get what they want. Usually in game design for RPGs, there being no predetermined "optimal path" forward turns into a quantum ogre situation-- it denies players the ability to make reasoned choices that affect the story. But when you change the mechanics they're engaging with from graph-paper mapping to threat navigation, you can introduce a different set of risks and tell certain kinds of emergent stories more directly.