Real places in RPG : Cats' Alley
One thing that is really often forgotten in fantasy gaming, is how compact medieval towns were. Streets were narrow (sometimes, like in this case, not even wide enough for a carriage to get through) and stone houses were very rare.
This is a picture of one of the few remaining very narrow streets in my home town of Troyes, Champagne Region, France. It is called "La ruelle aux chats", meaning cats' alley.
As you can see, the street is very narrow, the houses were not really stable, hence the beams connecting the buildings, to make sure they don't lean completely on each other (and don't crumble into the street either).
Since this is France, there is also a rumor saying the beams were used by lovers to get to each other stealthily...
Broad streets and luminous housing is not a typical concept in a medieval town. Space was scarce (at least if you wanted to live securely protected by the towns fortifications) and wattle & daub was the most used building material (at least in my home region). What is that ? It is a method for making walls, in which a woven lattice of wooden strips is daubed with a sticky material, like wet soil, clay, sand, animal dung and straw.
I guess some spells could help create or cut stones and make them affordable, but isn't magic expensive anyway? (I must say, a stonemason-wizard would look stylish, though)
Wattle & daub was quite cheap but had a tremendous disadvantage... It is quite flammable. The straw in the building material means that in a case of fire, a lot of buildings would burn down (this happened a few times in Troyes). The blaze will ignite other building quite quickly and fire spread easily and quickly.
Consequences on Fantasy
Well it means that uncontrolled open fire is to be banned, as much as possible.
If I were the burgomaster of a town, I would immediately ban all fire magic and either exile any petty offender, or just hang them high.
Imagine how many houses a fireball would set on fire! Remember, magical fire does not have to be a tamed resource, always in control of the caster and solely burning opponents.
Fire is non judgmental... Fire just burns everything it can: clothing, hair, buildings, cattle & mules, e-ve-ry-thing.
So next time your players are ambushed by assassins in a narrow street, jumping down on them from the house-stabilization beams, and defend themselves with a fireball or burning hands, don't forget to make them accountable for the destruction of at least a house block by the ensuing blaze.
Just in case you were running out of troubles to throw at your PCs.
That is something that would be absolutely banned from any urban area in a medieval setting. This alchemy arson weapon is MADE to be difficult to extinguish!
Guards would search persons for this kind of dangerous weapon and confiscate it at the town's gates!
You might even want to ban its components (probably a combination of pine resin, naphtha, quicklime, calcium phosphide, sulfur, and/or niter).
Wouldn't that be a wonderful opportunity for smugglers?
The ability to magically control fire (like the 5E cantrip "Control Flames") would be a must for a fire-fighting force in a city that can afford such a unit (most towns relied of massively levy the population in such cases). Having a sorcerer that can cast it would be nice, but having a wizard that would teach a unit of specialized guards this cantrip would be even better.
Not having a retired wizard adventurer on the roll could be a disastrous decision for a town.
If your party's wizard behaves badly or just plainly uses magic all the time, the town might want to have him sequestrated until he taught "Control Flames" to a bunch of illiterate town guards.
If you want your fire-fighters to be clerics or druids, the spell "Create Or Destroy Water" in its create rain version could be usefull and only is a lvl 1 spell. Still 10 gallons / 37 liters are not a lot of water to extinguish the blaze of an urban conflagration...
If you know of better ways to fight fire in a fantasy setting, please tell me so in the comments !