What prevents monsters from spawning in certain areas?

While digging a few "home holes" with doors for entrances ...etc. i noticed that after long hunting/looting/scavenging runs i come back home and some monsters spawn inside, what exactly prevents monsters from spawning in a cave/hole you turn into a home? lights ? tried that but doesn't always work :S any thoughts ?

 

i guess currently only you beeing there ... or a frendly NPC with weapons (in a city or a dungeon etc.)

 

No, NPC plays no role here. I saw a monster in someone's house. It wasn't hostile though...

 

hmm. i havent actually built anything on a planet yet. but.. in terraria w/e you had to build a back wall on your house to keep stuff from gettin in. and i think it had to be made of crafted building materials, not just a bunch of dirt. otherwise it just treated the structure like a regular ole part of the game world and let monsters in

 

I had a house before the first wipe and nothing ever spawned inside and I just built the house at the spawn (like right there where you beam down) since we can't move the beacon spot


Basically my house was a packed dirt house (the one that uses dirt to make brick like texture) and then I had it lit with torches, had a bed, crafting stuff, and doors to keep things out

nothing ever spawned so I'm guessing it was a combination of doors + torches + walls?

 

I'm going to assume that it has to have a back wall placed by the player and maybe a light source.

 

hmmm interesting , because i was starting to think since it wasn't player made materials (bricks ..etc.) that the game would treat it as a normal cave, granted most of the time i get passive monsters , but what if i get a herd of nasties ? lol

 

Or worse. Birds .