Why monsters started to spawn in my town?

[Poma]

I have a main base with 7 villagers. Recently monsters started to spawn there instead of usual critters.

https://i.imgur.com/piEa3ia.png

Why can this happen? My current progress is after mechs and before plantera.

[ToasterLlama]

is it a blood moon?

[Paulo Minoso]

put some background walls around the cavern to avoid enemies spawning idk why thats happening but they cant spawn if there are walls behind

[Poma]

No, no event is currently in progress, and this is already happening for a few in-game days

[i have no headphones and i must]

I think you need a properly lit area with walls. Don’t think the natural underground walls count, you need to put in your own along with maybe more torches

[Poma]

@pepega, as far as I know, towns with 3 or more villagers should change spawning rules to critters only, unless there is some event going on that overrides it. And it was working like that until yesterday,

[Halfshell]

Monsters can spawn on any wall that you didn't place. So you would want to tear down all those walls, enclose them in blocks for hallways and make sure they are lit. Otherwise its too easy to disable spawns so they make you do some work to make it "count".

[FlufyMarshMelo]

If there is at least 3 graves in the town it will become a graveyard which spawns low-level monsters.

[Poma]

Also it still works in my surface town - I didn't cover all the sky in walls, and it still spawns only bunnies and worms. So I think walls are not a requirement here.

[Poma]

I will check around for graves, although I don't see any graveyard effects or related monsters

[esotericist]

Is it an Expert or Master mode world?

Expert+ actually does not eliminate the chances for spawns around town, it just makes them less likely. But they can still happen and accrue.

[Poma]

Found the cause - crimson was approaching my base. It was still too far to change my current biome, but enough to enable enemy spawns. I've cleaned it all up, and now I'm again getting only fish and bunnies.

[esotericist]

That'd do it, yeah.

[Collateral damage]

The towns are poorly implemented and are very buggy. Monsters don't spawn relative to towns, they spawn on the edges of the screen. If you don't have at least 3 NPCs less than 25 blocks close to you, the monsters will spawn at the edges of your screen, regardless if there is a town there or not. That's also the condition for the music change. Of course if you step outside of town for a second, when you get back the monsters will not despawn and will enter the town. Sometimes they will still spawn even if you're in the middle of the town and can hear the town music, I haven't figured out why, likely a bug.
Of course, having a player-placed background walls helps, but that's unrelated to town mechanics.