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
Best Answer
You won't be able to prevent FBs from spawning. Basically, what I recommend is to think of, and treat, the caverns exactly like you think of the surface: as fundamentally an area outside your fortress that you need to manage your fortress's interface with in order to achieve security.
Then it proceeds naturally that the measures that you employ to be secure with dragons arriving on the surface are the same measures employed to be secure with flying FBs arriving in the caverns.