A hex crawl can be split into three main features. Most of the map is covered in hexes that contain quick encounters or short adventures. A few places on the maps contain complex, multi-level, multi-faction dungeons. Then there are the cities where the party can go to rest and recuperate between excursions. Cities are hardly a stranger to adventure and danger, though. In fact, cities usually concentrate crime and corruption just like they concentrate everything else. Put a hundred thousand people in close proximity to one another and the absolute number of thieves, assassins, vampires, and changelings will get high enough for each of them to have their own guild even if the overall density doesn’t budge.
So you’d expect cities to contain some adventure just like the wilderness and the dungeons, and the structure for that adventure is the urbancrawl. Just like hex crawling and dungeon crawling allow a party to explore wilderness and fortresses according to their own curiosity and courage (or foolishness, as you prefer), an urbancrawl lets them explore a city.
Urbancrawls are divided into neighborhoods, each neighborhood containing at least one city adventure, often several. In general, travel between neighborhoods takes no significant amount of time and aren’t impeded by random encounters. Players say they want to go to the docks and they simply arrive. Some specific encounters might trigger “the next time the players try to leave [neighborhood]” or “the next time players leave any neighborhood,” but these are rare exceptions to the general rule. Similarly, the basic layout of the city is immediately obvious. Players know what all the neighborhoods are called and what they look like from the start. There is never a question of terrain like there is with hex terrain, where players generally speaking have no clue what even the most basic features of any hex they haven’t been to will be. Traversing an urbancrawl isn’t about literally traversing the distance, it’s about finding needles in haystacks.
Each neighborhood has a number of encounters and adventures associated with it, and each one of those has a discovery DC. Players can roll Investigation to go looking for trouble in a neighborhood, and the DC they’re looking to hit is the lowest DC of any discovery DC in the neighborhood. If they exceed the DC by at least five points, they discover another encounter, provided they also meet that encounter’s discovery DC. For example, say there are three encounters in a neighborhood with discovery DCs of 10, 10, and 15. The party rolls a 15 on their Investigation check. They hit the lowest DC, 10, and they exceeded it by 5 points, so they discover an extra encounter, getting both the DC 10 encounters. They do not get the DC 15, however, even though they rolled a 15, because they’ve already discovered two encounters and they only exceeded the lowest DC of the neighborhood by 5 points. As another example, say the party is in a neighborhood with encounters at discovery DC 15 and 25 and they roll a 20 for their Investigation check. They find the DC 15 encounter and they have five points left over to discover another encounter, but since they didn’t meet the discovery DC of the DC 25 encounter, they don’t find it.
Discovering an encounter almost never reveals anything particularly alarming, but instead just reveals the hook for the adventure plot. For example, hitting the discovery DC means finding out there’s been a murder in the local flophouse, but doesn’t reveal the identity of the person who committed the murders, or it might reveal that giant vermin from the sewer are making off with people’s stuff, but finding out how INT 3 creatures the size of a warhorse figured out how to catburgle requires completing the dungeon that is their sewer lair.
Multiple encounters and adventures in an urbancrawl might all be related to one another, for example, a thieves’ guild might have lots of encounters relating to their racketeering, their dark magic item, the corruption of the town guard, their assassination gigs, and so on, spread throughout multiple neighborhoods. Opposition in the same sub-plot as already cleared adventures might start reacting differently, for example, the thieves’ guild might increase security after a few of their operations are shut down and they realize someone’s after them. If the party isn’t particularly covering their tracks (or especially if they’re bragging about putting a stop to crime), the thieves’ guild might send assassins after them.
There might also be some adventures in a sub-plot with absurd, 35+ discovery DCs, but which can still be located through the other adventures in that sub-plot, for example, the thieves’ guild lair might be extremely well-hidden, but interrogating captured thieves might help the PCs discover it, or you can use the three clues method for a mystery adventure, only instead of placing the three clues that lead to the thieves’ guild lair all in the same adventure, you space them out between several, which means the players will probably have to complete multiple thieves’ guild adventures before they stumble across one of the clues leading to the hideout. Additionally, finding and defeating the leader of a sub-plot antagonist organization, like capturing or killing the head of the thieves’ guild, might automatically clear some or all of the remaining encounters or adventures in the sub-plot.
Megadungeons, Megacities, and Megamysteries
Megadungeons, megacities, and megamysteries are three campaign structures too straightforward to require complete articles, so we’ll squirrel them away here after the explanation on urbancrawls. A megadungeon is just a single dungeon made so unspeakably vast as to be able to sustain an entire campaign by itself, and a megacity is the same thing applied to urbancrawls. In both cases the only advice particularly needed is already included in the Art of Adventures. In the case of a megadungeon, just make sure you maintain enough variety in both opposition and layout that it doesn’t get dull.
In the case of a megacity, you’re using an urbancrawl to string together lots of encounters and adventures just like with a hexcrawl, and unlike with a hexcrawl there’s no particular process for moving from one location to another. Make sure you have enough content to last 13+ sessions, create a city with enough neighborhoods to contain that content without getting more than 3-4 adventures deep in one neighborhood, and that’s it. Creating that content is obviously not a small task, but it’s all encounter and adventure design, the campaign structure is only slightly more complex than a serial campaign.
A megamystery requires only slightly more elaboration. A mystery campaign is built on the same three clues principle as a mystery adventure, but rather than having the clues contained in a single adventure, some scenes have the clues to them spread throughout multiple adventures. The final scene of an introductory adventure contains three clues, each one pointing to the introductory scene of a completely different mystery adventure. These adventures, in turn, contain more clues to other adventures (some in their final scene, some in other scenes). So long as each mystery contains at least three clues each pointing to other mysteries, and each mystery has at least three clues point to it from other mysteries, it can be reasonably safely assumed that sooner or later players will find all of them. Just like with the standard three clues method from the adventure, if the players miss all three clues, new clues can be produced by having the villains advance their agenda in some way that leaves a new mystery behind, with this mystery containing clues to the start of whatever mystery the players have missed. If this happens often enough, the villains might advance their agenda to the point where they actually succeed in whatever dark ritual they have cooking up, thus prompting the boss fight with Cthulhu at an earlier (perhaps much earlier) level than was planned, which would probably end poorly for the players.
Players need to be prepared to take lots of notes for a full-on mystery campaign, both because this will help them catch clues and more importantly because it will help them keep track of the leads they’ve already found. A single mystery contains clues pointing to as many as three other mysteries, and if players manage to find all three, they could quite possibly forget one of them by the time they’ve solved the other two, especially if they discover even more mysteries while investigating those two, and follow up on those before going back to the one leftover from the first set.
The main question for a mystery campaign is how to structure the ending. You can always just have the climax mystery be sitting on top of a large tower of mysteries. The intro mystery has clues leading to mysteries A1, B1, and C1. A1 has clues leading to B1, C1, and A2. A2 has clues leading to A3, B2, and C2, and so on, just like with a mystery adventure. This is a perfectly reasonable way to build a mystery campaign if you don’t mind risking that a lot of your work will be missed and that at least some of it is virtually guaranteed to be missed. It’s unlikely, but entirely possible, that players will go from the intro to A1, to A2, to A3, to the climax, completely skipping over all six adventures in the B and C columns, over half of the work you’d done. It’s quite likely that players will get through at least one level of the tower without completing all three mysteries on that level (i.e. they’ll do A2, get a clue to C2, then from C2 find the clue to C3 and go up to the third level of the metaphorical tower without ever completing B2).
One way to solve this is by making the goal to solve every mystery, not just the topmost one. For example, rather than the goal simply being to find the head of the cult and stab him before he can summon Cthulhu, perhaps Cthulhu’s dreams influencing cultists and monsters and spawning cyclopean horrors to bring about his own awakening, which means there is no head of the snake to cut off (technically there is, but it’s Cthulhu, who is a high-level encounter if he’s beatable at all), so the players need to put a stop to each cult and star spawn and so on trying to awaken Cthulhu in various different ways. This issue lacks a final confrontation with any kind of central antagonist, but if that’s not something you and your group need for a satisfying conclusion, this method works perfectly well.
Another way of solving the problem is by having the mysteries be an interconnected web but the opposition more linear. For example, the one mystery is about discovering the location of Rlyeh and stopping the cultists from going there to wind up Cthulhu’s alarm clock, another is about preventing them from performing a mass ritual sacrifice when the stars align, a third is about stopping them from recovering the Necronomicon and reading from it the magic spell that will awaken him (I’m aware that I’m mangling Mythos lore a bit here, but translating Lovecraft faithfully into D&D is not the point of this example), but then the actual specific cult lieutenant attempting to complete these rituals happens in the same order no matter what order the players discover and investigate the mysteries. The first one the players investigate is always a goblin cult, the second one is always a vampire and his spawn, the third one is always a mind flayer and his thralls. This has the disadvantage that you can’t make any of your clues have to do with the antagonist of a mystery because the antagonist is undefined until the players begin to investigate. You can’t have the players learn about the cult’s trip to Rlyeh by reading correspondence where they talk about how the mind flayer is off to Rlyeh because maybe the mind flayer is actually the one chasing the Necronomicon. The advantage, however, is that the final confrontation will always be with the primary villain.
You could also have the final confrontation trigger after all mysteries are solved and be some other sort of adventure entirely. Maybe each cult lieutenant holds one fragment of a key to Rlyeh, and the last adventure is a dungeon crawl through the primordial city. This method can be done in stages, so that one web of mysteries, after being entirely cleared, leads to a wilderness journey, which in turn leads to a new intro mystery that leads the party into a new web, thus providing the occasional non-mystery adventure for variety.
Finally, you can always just ask your players to play along. “Yes, you’ve found a lead on the head of the cult, but you also have three other leads you haven’t followed up on, and you’re going to need to chase them all down eventually, so how about we leave the final boss for the end?” This has the disadvantage that it’s a really out of game reason to put off the head of the cult, but if the other methods all require drastic overhauls to your campaign that you think will do more harm than good, there’s nothing particularly wrong with using this option.