Biomes
Biomes define of which type a level is and what enemies are allowed to spawn.
Biome Types
All biomes are "just text", so you can make your own, following are the ones the unmodded base game consists of.
Keep in mind that upper/lower case is important and all-lower case is the default for our game files.
Primary Biome Types
land
water
air
ice
Secondary Biome Types
forest
desert
underwater
mushroom_forest
airship
xmas
valentine
What's the difference?
Technically there is no difference between Primary Biome Types and Secondary Biome Types, but we use them internally to mark levels.
The Desert level has land and desert as biome.
So all enemies that can only spawn in the desert biome but not in the forest would have only the desert biome set, but not land.
An enemie that should span on any land can just have land set as its biome.
Example
For example see Steppi, it can only spawn in the desert.
Compare that to Dog, who can spawn on any land and airship, but not in any other air level, because it can't fly, but he makes sense in the airship level.
Dog | Steppi |
|---|---|
 |  |