My players are soon to reach the city that houses their Tier 3 BBEG and his laboratory is in the abandoned section of the city's sewer underneath his old mansion. However, only just now, after having it all planned out in the story do I realise I have no idea how sewers work or look or how they're structured. I have googled and found this discussion and this one and even this one too that were all helpful, but Im wondering what advice you have? Maybe personal experiences in creating a sewer system in your campaign? Im all about the concept of verisimilitude to influence narrative, trap or encounter design, rather than suddenly a meat grinder appears in a standard urban sewer haha.

In case it helps people with their responses, its a party of six level 10 - 11 mix of veteran and completely fresh players, (Thief, Vengeance Pally, Div Wiz/Temp Cleric, Wild Mage, Bloodhunter, Moon Druid) in a Gothic Survival Horror setting, carried on after a successful Curse of Strahd campaign. This is the map of the city:

Oh wow, a discussion that suits my career! 

As a civil engineering draftsman, most of what we do is plot which way the water needs to go. We do that by using low points known as “Sags” to channel said water. There are two main types of sewers in most cities in America today, the storm sewer system, and the sewage system. There are also grey water systems in some places, but for our purposes we’re lumping that with sewage systems for now.

Sewage has smaller pipes, less access points, and more stink than Storm Sewers. These are the elements of waste, restaurant refuse, and biological matter small enough to go down the drain.

For adventure purposes it’s always assumed to be Storm Sewer, the pipes are large, there are many access points, and they aren’t full of poop. 

So, these tunnels work by letting water flow through them, easy enough right? Have the tunnels run from the uphill side of town, to an abandoned rock quarry or other large hole in the ground. The tunnels will occasionally meet in what’s called a Gallery, and the outflow pipes will be either higher or lower than the inflow pipes. This is so the output can be timed, for higher pipes, or higher pressure for lower ones. 

Think of it like all the rainwater needs a place to go, put manholes at every major intersection and water intakes on high traffic corners. Maintenance entrances will be above galleries, typically on the highest sections of the uphill side. The tunnels will run in more or less straight lines from the first gallery on the hill, to the quarry on the other side of town, with a space between them of about a city block. Cross tunnels will be rare and have heavy doors between them that the water pressure can open, or hold closed. There typically are not maintenance rooms underground unless they are made watertight, but those are the best place to put your BBEG. 

I hope that bit helps, but you can google the Storm sewer layouts of city projects in places like Dallas, Houston and Memphis for better ideas on how they are set up. 

good luck!

What a brilliant answer! I learnt more usable and understandable advice from paragraph 6 & 7 than I did in two days of research. Thank you so much!

Bear in mind that modern cities are much bigger than most fantasy cities, and pavement is much more efficient at directing all the water into narrow channels than likely fantasy equivalents, which means they're dealing with a lot more water, all of which means the drain system for a normal sized fantasy city won't be big enough for humans to practically get through unless there's some reason other than actual transport of water they need to be bigger.

There's something to be said for hiring kobolds as sewer workers -- they're decent miners, they can fit through much smaller spaces than humans, and they don't need light This will result in the fun of adventurers crawling through 4' tunnels. Gnomes also work but are probably more expensive.

As stated above, high elevation to low is key.

I see that you have a river there, which would be where most of your outflow would be going to and away from the city.

Additionally, most large sewer systems would follow the roadways you've got there on the map with smaller pipes/lines branching to buildings and other areas that the players would not be able to traverse.

Smaller side-roads would not be as large (though potentially still traversable) and most, if not all, sewage lines from buildings would be simple pipes that only a rat or such things would be able to crawl through. <However, this need not always the case as some buildings/homes could have sewage access points in their cellar. But that would certainly not be normal and probably done for less than legal purposes. Typically it would be a small hole or something to throw a chamber pot down (if at all); but could be widened after the fact to allow someone to enter the sewers that way.

1st Step: Make major roads (though you don't have to make all of them) walkable tunnels.

2nd Step: Pick areas along the river line to place your large, main water outflow tunnels. These can be access points if you want. Easiest thing to do would be to follow roads that lead to the river and have the outflow pipes at the end there.

3rd Step: Figure out where you'd put manhole covers and/or drains on or near the streets. (as stated above again) Major traffic areas (large roads etc.) would be given the most priority for drainage and upkeep. These drains would be in pretty regular intervals and also act as potential light sources in the darkness below. Other than that, most intersections would have drainage grates, but access points would/could be limited to every other intersection or less.

4th Step: Following your map above, you know where your final destination is; either follow the maps road network, or (using it) make it a confusing mess. <Who knows, maybe the urban planner was drunk... like so many in the real world.

Natural/structural Hazards that you can use to guide players towards a area and/or to make a streamlined design into a maze:

Collapsed Areas: Old and abandoned sewers around the mansion could have collapsed regions and/or areas that are able to collapse and be a hazard. This would also bar them (possibly) from going in the wrong direction. Don't just have it scripted though; there has to be some reason (player-made reason) why the things would collapse now rather than a day later or before.

Cisterns/galleries: These would be large open pits and areas. Some holding flowing/stagnant water within them making them great areas for either blocking players or staging a fight scene. <Giant Rat nest or slimes or something living in them kinda thing. Galleries (never knew they were called that) also have other tunnels that join them and can make decision making a bit more of a thing. Wrong turns and all that.

Sealed doors/gateways/bars: Not all of the tunnels need to be open and accessible. If there is a area you don't want your players to go to, seal it off with a iron bulkhead or bars. Alternatively, these could also be used as hurdles for them to get over or through...

Tunnels that get smaller or lead to smaller pipes: another potential "go no further" option is a tunnel that gets smaller as it goes or dead-ends to a much smaller tunnel or pipe.

Debris Clogging up a line: this one's pretty simple. A pile of debris blocks a tunnel and makes passing it impossible. Normally it would be cleaned, but hey it's been a while since someone's been down here. <Also a good opportunity for a Investigation/perception roll to find a unique item that might have washed down here.

Slick drainage slopes and shier walls: As per the initial thing of "High to low", There are plenty of slopes that could force a Dex save or check. In addition, there can be clear drop-offs leading much deeper areas below the ground. If you start from the top, you have to worry about sliding down these things and potentially plunging into some pit of "water". If starting from the bottom, you may find yourself having issues getting to the top...

Fast-flowing or Static water: As always, there is going to be water present. Some of it could be a hazard that sweeps a player away, maybe even a whirlpool that leads to certain death if you fail a athletics check. Then there's the nasty stuff that leads to diseases later on if you fail a Con save.

Ultimately, Sewers should follow a rather direct path with decently-planned layouts following the roads above them. However, if you are looking to make a maze or get crazy with the layout, there's nothing saying that generations of builders wouldn't integrate older parts to newer. So, you could end up having what is effectively a labyrinth below the city.

Quote from Seiys >>

As stated above, high elevation to low is key.

I see that you have a river there, which would be where most of your outflow would be going to and away from the city.

Additionally, most large sewer systems would follow the roadways you've got there on the map with smaller pipes/lines branching to buildings and other areas that the players would not be able to traverse.

Smaller side-roads would not be as large (though potentially still traversable) and most, if not all, sewage lines from buildings would be simple pipes that only a rat or such things would be able to crawl through. <However, this need not always the case as some buildings/homes could have sewage access points in their cellar. But that would certainly not be normal and probably done for less than legal purposes. Typically it would be a small hole or something to throw a chamber pot down (if at all); but could be widened after the fact to allow someone to enter the sewers that way.

1st, 2nd, 3rd and 4th Step: <paraphrased>

Natural/structural Hazards that you can use to guide players towards a area and/or to make a streamlined design into a maze:

Collapsed Areas: <paraphrased>

Cisterns/galleries: <paraphrased>

Sealed doors/gateways/bars: <paraphrased>

Tunnels that get smaller or lead to smaller pipes: <paraphrased>

Debris Clogging up a line: <paraphrased>

Slick drainage slopes and shier walls: <paraphrased>

Fast-flowing or Static water: <paraphrased>

Ultimately, Sewers should follow a rather direct path with decently-planned layouts following the roads above them. However, if you are looking to make a maze or get crazy with the layout, there's nothing saying that generations of builders wouldn't integrate older parts to newer. So, you could end up having what is effectively a labyrinth below the city.

 Oh goodness!!! Thank you so much! These are all amazing suggestions and super helpful. Much appreciates.

Also consider that sewers are not the oldest thing, but building underground is. There is every chance that what is now the sewers used to be catacombes, or escape tunnels, or half-finished siege tunnels from when the town was smaller, abandoned mines, natural caverns...

The sewer I'm working on for my world is built around an underground river system, which justifies why there is space to move around in the caves & sewers around it (rather than just small pipes!)

You don't need to make these things realistic, and for the sake of gameplay, it's usually better if they aren't.

The players are going to be interested in what they can immediately see, smell, and touch. You don't need vast networks, it's not all that important that there are logical in-flows and out-flows - I'd advise focusing on the specific parts that the players are going to travel through and make them exciting. Your players will appreciate an encounter where they are followed by an eyestalk that keeps popping up from the filth and eventually erupts as a tentacled monster far more than they will complex maps of places they'll probably never go to, logical water flow systems etc.

I'd advise that if you're making the sewers a cool location, then focus on:

• What denizens dwell there? A wererat gang of thieves? Drow who've broken upwards from the underdark? How do those inhabitants change the sewers? How do they use them to get around? Do they have rafts?
• Could the sewers have been something else once? What if they're actually repurposed burial catacombs? If they are, what do the restless spirits think about that?
• Are the sewers hiding any secrets to uncover?

Consider that in Curse of Strahd, which you've just played, there are barely any agricultural workers. Almost everyone lives in a village or town; how do any of these people get fed? You need 5 people working in the fields for every 1 person living in a town to keep the economy going. The only workers seem to be the Wizards of Wine. But does this matter at all? I really don't think it has ever mattered to anyone playing that module. Focus your energy on the cool stuff.

Sanvael has a good point. Perhaps the best thing to do it plan the section of sewer you want them to adventure in, then map it under the city so that it flows in a sensible direction. Then block off the rest of the pipes and leave them boring and full of turds so the players have no interest in going there!

Quote from Sanvael >>

You don't need to make these things realistic, and for the sake of gameplay, it's usually better if they aren't.

The players are going to be interested in what they can immediately see, smell, and touch. You don't need vast networks, it's not all that important that there are logical in-flows and out-flows - I'd advise focusing on the specific parts that the players are going to travel through and make them exciting. Your players will appreciate an encounter where they are followed by an eyestalk that keeps popping up from the filth and eventually erupts as a tentacled monster far more than they will complex maps of places they'll probably never go to, logical water flow systems etc.

I'd advise that if you're making the sewers a cool location, then focus on:

• What denizens dwell there? A wererat gang of thieves? Drow who've broken upwards from the underdark? How do those inhabitants change the sewers? How do they use them to get around? Do they have rafts?
• Could the sewers have been something else once? What if they're actually repurposed burial catacombs? If they are, what do the restless spirits think about that?
• Are the sewers hiding any secrets to uncover?

Consider that in Curse of Strahd, which you've just played, there are barely any agricultural workers. Almost everyone lives in a village or town; how do any of these people get fed? You need 5 people working in the fields for every 1 person living in a town to keep the economy going. The only workers seem to be the Wizards of Wine. But does this matter at all? I really don't think it has ever mattered to anyone playing that module. Focus your energy on the cool stuff.

 Hmm yes. Quite wise. These particular players really resonate with the open world, sandbox feel that CoS offered but I was kinda always going to go with the illusion of choice (closed off causeways, funnelling them certain places) anyway. I guess I wanted to know how a sewer works/looks like properly, before I then mould it down for a DnD map/series of encounters.

In my recent campaign, the city has a sewer system being used by thieves to carry out their heists and serve as a base of operations. As part of this, the city itself has a clan of kobolds who serve as Sewer Union 405.  They make sure the sewers stay in good shape and also serve as exterminators for critters that may move into the sewers.  As a result, they have asizable number of traps in place throughout the area to catch any problem creatures.

Thank you all very much! Using the info you've all kindly assisted me with here, I've been able to wrap my head around the planning of the sewer MUCH more. I've got a few locations prepped in my head-cloud, including a beholder den in a Gallery. The attached image is my draft battle map for the party's Tier 3 BBEG lair beneath his old home. I know it doesn't fit the confines of a real sewer design, but what do you think about this sewer maintenance/monitoring/viewing chamber? I've got other files that I can't upload to this design site (Terrain Tinker) like pipes, vats, alchemy tables, tesla coils, etc. I also intend to have a cat walk from the upper most platforms on the left and the right connecting each other and the walls are imaginary, reaching as high as the highest visible piece of terrain. So, other than scatter, what do y'll think? The other layers appear here.

Fun fact: sewers are weirdly anachronistic. In Europe, Sewers were, by and large, introduced in the mid 19th century and were massive infrastructure projects. If you are worried about realism, the answer to how you should build sewers is : you shouldn't. You might as well include a subway line. Even if you did, most sewers were hardly transverable, but represented closed systems of pipes.That said, this might be one of the situation where historical accuracy maybe should take a back seat to fantasy.

Quote from Verenti >>

Fun fact: sewers are weirdly anachronistic. In Europe, Sewers were, by and large, introduced in the mid 19th century and were massive infrastructure projects. If you are worried about realism, the answer to how you should build sewers is : you shouldn't. You might as well include a subway line. Even if you did, most sewers were hardly traversable, but represented closed systems of pipes.That said, this might be one of the situation where historical accuracy maybe should take a back seat to fantasy.

 Yeah after all my research and suggestions here, my usual love of basing my world building even partially on verisimilitude is out the window. hahaha only once I committed to that was I able to put this together, pushing through my retentiveness hahaha. Thanks mate.

Far as I know most fantasy sewers are inspired by the sewers of Paris, possibly mixed up with the catacombs (Paris has a lot of underground stuff, apparently because it used to be a place they mined limestone), and while some sort of sewer system dates back to the 14th century, probably more inspired by the 19th century version. It looks like ancient Rome also had a sewer system, but doesn't seem to have been the sort of labyrinth Paris had.

In medieval times, sewers were more of open ditches or rivers, there wasn't that much of what we'd imagine a sewer.  If you want some inspiration for a sewer system with goblins and a goblin pirate ship and a deadly trap watch Goblin Slayer S1 E8 and S1 E9.  It's a good inspiration for a sewer run, plus with how goblin slayer fights, its always interesting to see HOW they get out of it.

Most fantasy worlds may seem like they are based on ancient times, but most have ancient lost civilisations, as well as far better governments than ancient times. If you assume that the world blinked back to the stone age right now, by the time they reach D&D times, they will still have the sewers we built now, albeit very derelict. If we uilt them from stone and not el-cheapo concrete, then they would easily last.

So if an ancient civilisation built a city, then was killed/lost/magicked into trees/ whatever, then the next civilisation who builds there will have sewers. And they will probably build sewers themselves.

Also, Rome had sewers in 500BC. So just because the England (medieval england is the main basis for these things) was gross doesn't mean the whole world was!

I build sewers into towns as I base my worlds on the idea that people stopped being obsessed with technological advancement because magic could do that. Nobody invented guns, because the Catapult spell can do that. Nobody invented aeroplanes because flying brooms and carpets can do that. But if you think people who can do magic will be sitting around using prestidigitation on your leavings? Nope, someone invent a pipe to take this stuff to the nearest river!

We lived in medieval times for about 800 years, which is D&D terms is an elderly elf. So if you've got a world which has history stretching back into multiple generations of elves, then these people may have been living in medieval times (IE not gunpowder) for several times that - long enough to invent a fake river to poop in!

I'd like to think that (at least for larger cities and/or people willing to fork over the gold to pay their price) they could hire some people skilled in the arts of magic.

Stone Wall (with a bit of creativity and a skilled practitioner of magic) can easily create large cavernous sewer systems.

Mold Earth is a cantrip of all things, so even a novice mage looking for some work could possess this one. And, with enough time, easily create a city-wide sewer system that is traversable. 5ft cube of dirt every 6 seconds. <I would personally consider this single spell to be the primary source of underground tunnels and sewers.

Point being, magic makes such large construction efforts rather effortless. That can be a good thing and a bad thing. Creating a underground sewer could be a organized/official project with some actual engineer aiding the mage to get a desired result, or be created by some rookie mage looking to make a quick buck and possessing zero concept of how a sewer system should be.

And on top of this, as ThorukDuckSlayer stated above, the worlds that D&D take place in often have ancient cities buried beneath other ancient cities. People created a city, got wiped out or left for some reason that's been forgotten, and another civilization found what was built and used the same location to put their own city. Then, possibly, that city was destroyed and another was placed on top of it.

Underground tunnels might be a huge project for us in this world, but for something like D&D, it could honestly be just some bored mages hobby (that's you DM's) to create sprawling tunnel networks for nothing more than the lol's of watching people try to navigate them later.

So I have just started running a sewer adventure for my players, our party is 60% TOTM and we only use physical maps for combat that is more complicated it. 

Therefore I didn't bother mapping out the whole sewer system at all physically. I first of all created a set of potential encounters then decided which needed mapping out, the rest of it I then added in description and narrative to describe the passage through the sewers, things they saw, experienced, sounds and smells. 

This meant I could describe miles and miles of sewers and take them on a journey through the underneath of the town without needed to map it all out. 

Quote from Scarloc_Stormcall >>

So I have just started running a sewer adventure for my players, our party is 60% TOTM and we only use physical maps for combat that is more complicated it. 

Therefore I didn't bother mapping out the whole sewer system at all physically. I first of all created a set of potential encounters then decided which needed mapping out, the rest of it I then added in description and narrative to describe the passage through the sewers, things they saw, experienced, sounds and smells. 

This meant I could describe miles and miles of sewers and take them on a journey through the underneath of the town without needed to map it all out. 

 Yes, this is essentially what Im doing now too. :)