On Sewers in Cities

The nagging question that hangs over the head of anyone running or designing a mid-to-large-sized city is what to do if a character wants to head into the sewers? Unlike dungeons, an OSR best practice for running sewers doesn’t exist. Today, we look at which levels of abstraction can be used and at some new procedures I developed from this exploration.

Looking to the Past

Designing a city has led me to dig through my favorite city settings, such as The City State of the Invincible Overlord, Blackmoor, Zyan, Sanctuary, Punjar, and TSR’s many famous cities. And notably, TSR’s take on Lankhmar in Lankhmar City of Adventure. It is a city defined by its sewer-dwelling factions, but it avoids detailing the sewers entirely. It wasn’t until the release of Swords of Deceit, a full year later, that GMs were provided with a map for the sewers. In Waterdeep and the North, released in 1987, we get a similar map that outlines the primary and secondary passages in Waterdeep. And, Greyhawk isn’t detailed until the box set in ’89.

This observation was interesting to me because, until 1986, it appears that two levels of abstraction existed for the regions below a city. The first was a complete abstraction, and the second was defined dungeon spaces with keyed rooms and details. The sewers were either undefined or completely distinct. As with most things, something in the middle is probably ideal; thus, my exploration of different levels of abstraction available to the GM for sewers.

Image from Greyhawk & Waterdeep: A Tale of Two City Sets (1989, 1994).

Mapping Design Space

In my day job as a brand/UI designer, I often employ a spectrum-based approach to mapping out new design space. While doing this, concepts are pushed to their extremes, with distinct steps defined within to explore. The process produces outputs that can be easily compared to debate the pros and cons of each and find a solution appropriately scaled to the challenge at hand. This process felt like the natural way to approach defining a level of abstraction that felt uniquely testable at the table. Presented here are the levels as I defined them, and my findings — listed as pros and cons. Skip to the bottom if you just want to see the procedure I came up with.

Option 1: Complete Abstraction

The space is described, and travel time and distance are mostly ignored, with the GM choosing to make or not make random encounter checks. 

Pros:

1. Speed: I would consider this the primary pro, where travel through the sewers doesn’t slow down the pace of play, especially if the session is crowded.
   
2. Minimal prep: In the absence of any prep, you can always think up a random encounter on the fly if the check comes up, or this is as simple as a list of random encounters to check against.
   
3. Ease of play: because there are no new procedures to learn, player friction is absent.
   

Cons:

1. Trivializes travel: sewers are a space that is likely confusing to navigate, and it fails to emphasize the increased danger they might pose compared to normal city travel.

Option 2: Mechanically-assisted Abstraction

The best example of this is the procedure I riffed on for my Cy_Borg campaign, by Craig Payne of Exploring Infinity, titled “Building a Better Labyrinth – A Maze Mechanic Idea,” in which a deck of playing cards is used to represent travel through the sewers. In systems where skill challenges are commonplace, they would also fall into this category. 

Pros:

1. Player involvement: while true player agency may be limited, there is still a sense of involvement.
   
2. Minimal prep: you are probably using a system-driven tool or one developed for you, so the prep is likely as simple as modifying encounters to fit your setting. You might even have a few modules with encounters to pull from.
   
3. Ease of play: because there are no new procedures to learn, player friction is absent.
   

Cons:

1. Feels like a sub game: If a mechanic is too complex, it may feel like stepping out of the world to run a set of mechanics, and…
   
2. Risks feeling repetitive: where the labor of the tool supplants the mind’s eye.
3. May require more tools: in my examples, these would be the skill list or the deck of cards.
   

Option 3: Partially Abstracted Space

Using a point crawl or defined regions, this approach relies on a map with abstracted passages or spatial representations. Encounters are checked based on distinct increments traveled, i.e., nodes or regions passed.

Pros:

1. Speed: There is no setup required to enter the sewers, and they can be run quickly without additional work from the players or GM.
2. Player involvement: While true player agency may be limited to selecting a simplified path, there is still a sense of involvement.
   

Cons:

1. Heavier prep: requires a city map with regions or points representing the sewers before entry, and a full table of random encounters.
   
2. Lacks exploration: the level of abstraction is still high enough that characters are really only passing through the sewers, not exploring them as a place.
   

Option 4: Low-fidelity Mapping

Common to the AD&D modules listed above, a representative system of tunnels (normally in loose scale) is overlaid on the existing city map. 

Pros:

1. Ease of play: the sewers can be mapped and run similarly to dungeon passages.
2. Higher player agency: travel decisions, such as which direction to head, are left in the hands of the players.
   
3. Ease of play: can be mapped and run in the familiar style of a dungeon.
   

Cons:

1. Heavier prep: if your city setting doesn’t already provide this map, you will have to create it, along with a table of random encounters.
   
2. Travel distance is unintuitive: which makes knowing when to check for random encounters a little more difficult.
   
3. Can be hard to use: The complexity of the sewers in a larger city can be visually busy, especially when referencing printed materials at a smaller scale.

Option 5: High-fidelity Mapping

Falling just short of a complete dungeon map for each of the sewers is a high-fidelity map with abstracted spaces added on top. For example, take this snippet from the 3.5 version of The City State of the Invincible Overlord by Necromancer Games.

Pros:

1. Ease of play: the sewers can be mapped and run in the familiar style of a dungeon.
2. Normal player agency: choices presented to players are concrete and familiar. 
   

Cons:

1. Heavy prep: requires the GM to have a key for the map on hand at all times and, most likely, to have it fully developed before the campaign begins.
   
2. Cumbersome: The sewers must be mapped or referenced from a map in context with the city above, which may require finding maps or rearranging the table when in person. 
   

My New Procedure

Following this analysis, I brainstormed a set of procedures for sewers in OSR systems that I have been testing. It falls within my defined spectrum between options 3 and 4, a hybrid of the low-fidelity map system with purposeful use of abstraction for frequently traveled sections. I determined this to the ideal balance of speed factors to fit my style of OSR play. This method also encourages the characters to explore the sewers by rewarding them with fewer random encounters or swifter travel — a representation of familiarity with their surroundings. These procedures have also been designed to be as system-agnostic as possible. 

Pictured here is the sewer map I constructed for a DCC Lankhmar campaign I am running as a reference.

The sewers (from the AD&D module) have been superimposed on top of the city map, and a loose grid has been added to measure distances quickly. The large lines are major sewers, the thinner lines are offshoots. The dots at the end of lines represent access to the surface. Grayed-out boxed areas are larger random encounters (in places like service areas and junctions), and the grayed-out dotted outline is a planned dungeon in my campaign. The numbers are keyed locations on the surface and are unimportant for this exercise, but are useful for locating the party’s position on the map.  

How to run the procedure.

1. Establish whether the characters intend to explore the sewers or simply use them as a means of travel.
   
   1. If traveling: go to step 2.
      
   2. If exploring: go to step 3.
      
2. Have the characters traveled this route before?
   
   1. If yes: abstract travel, describing it as a familiar path and checking for random encounters every 8 squares, a minimum of once.
      
   2. If no: go to step 3.
      
3. Determine if the characters are familiar with this branch of sewers.
   
   1. If yes, check for random encounters every 4 squares (representing faster travel).
   2. If no, check for random encounters every 2 squares (representing slower travel).
4. Describe cardinal directions and branches using rough distances (standard dungeon mapping procedure).
   
5. Run any larger encounters or dungeons encountered. (optional; include only if part of your city setting).
   

All of the numbers in this procedure are easily adjusted to the scale of your city, flavor to taste. Also worth considering is whether the characters are being chased; in cases where they are rushing through the sewers and making even more noise than usual, encounter frequency can be increased as well.

How to prep for this method.

1. Prepare the sewers map.
   
2. Prepare a random encounter list for sewer tunnels.
   
3. Prepare a random encounter list for larger encounter areas (optional, can be excluded if these areas are not a part of your city)
   
4. Collect reference modules for any accessible dungeon areas (also optional, and can be excluded if not a part of your city)
   

The benefit of this approach is that you can scale up this work by starting with only the first two items, which can be prepped in a couple of hours or less. As long as a map of sewer lines exists, larger areas and dungeons can be added on top at a later date — possibly found via secret passage, future construction, or the surface.

Looking Forward

I am actively testing and editing these procedures in my games. Check back for updates.