Monster spawn conditions

[BJDware]

Is there any detailed information out there concerning the spawn conditions of Monsters?
This is not in regards to monster proofing area's, but in regards to farming.

I recently leveled out a Biome with the intent to farm the ground monsters and tree's. But now, the monsters no longer spawn.

I Leveled out the hills, and filled in any holes, ditches etc with dirt. I Even filled in some of the underground area's to reduce spawns underground instead of the ground level.

Any help in regards?

[Astasia]

They don't spawn in front of backwalls you placed yourself or are made out of non-natural materials. IE a random dungeon that spawns with a brick backwall will have no random monsters spawns in it, but a random dungeon with a dirt backwall will. If you place your own dirt backwall no monsters will spawn in front of it, I believe. Light level has nothing to do with it. I don't think the foreground block matters at all either, IE they will spawn on top of any type of placed block. That last part may or may not have changed at some point, I don't know, I haven't looked it up on the wiki in a while.