I want to wall off the edges of the first cavern layer to prevent Forgotten Beasts from spawning there. I've observed that after I built a wall around the first set of walls that creatures starting spawning on top of the walls. So do Forgotten Beasts spawn at any of the following places on the edge of caverns:
- On top of man (dwarf?) made structures
- On top of surface of water
- Underwater
- Open Space, for flying Forgotten Beasts
