British Isles (England, Scotland, Ireland, Orkney Islands) | The History of Sanitary SewersThe Orkney Islands are the location of excavations that show drainage systems dating as early as 3000 BCE. Lavatory-like plumbing systems were fitted into recesses in the walls of homes, with drained outlets, and certain liquid wastes were drained to area(s) either under or outside of buildings/homes.
One Sorcerer/Druid/Hedge Wizard with the Mold Earth cantrip can just dig up whole irrigation systems without breaking a sweat!If your town is not to far from a major city, perhaps the sewer beneath it is part of public works to take waste and greywater out the city and into the country to be used in fields. Or maybe there are wealthy aristocrats who had sewers built in the town. Or maybe with the help of magic, the public works are more practical in smaller towns than they were in much of the real world until the industrial age.
Maybe there are natural caverns under the town. The "powers that be" only had to dig a few sink holes, in and there, to let the water drain. If this is new work it could have angered the "residents" of the cave or revealed a nest of spies, a small thief guild or an ancient treasure...This is a silly setting problem to have, but it's recently come up in my megadungeon game. Or more specifically, in the small town beside the megadugenon. How do you make sewer dungeons believable?
To the best of my knowledge, medieval sewers were little more than open air ditches. Rome's Cloaca Maxima is a good model, but it's the sewage system of a world capital. I doubt that my little dungeon-adjacent village could justify such an engineering project. So when you're in a village setting rather than a major city, how do you justify sewer dungeons? Or are you better off inventing some other kind of subterranean labyrinth and saving yourself the verisimilitude hassle?
(Comic for illustrative purposes.)
Reminds me of the urban legend of Giant Alligators in the NYC sewer system from people flushing them down the toilet.You know, when I first glanced at the thread title I thought it read:
How do you make sewer dragons believable?
And thought to myself, "What is sewer dragon? Some cross between a dragon and a crocodile or something?"![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.