And i kinda figured out when your caverns start to spawn moss and fungus. It means that natural caverns are really, really close to the surface. Withing 15 layers every time so far
It really sucks for early expansion, since you cant dig too far down, so you have to make a horizontal fort, not vertical, and occasionally you'll actually lack stone for your constructions. I've always run into enormous deposits of magnetite in such cases, providing astounding amounts of iron.
the benefit is easy. Screw above ground. Build a Battle Tower above your entrance. All animals are safe inside. By excavating vast sub terran caverns with sandy or clay soil, you get your own nice tree industry, safe from invaders. If you brought some birds, you wont need to fish at all, since 3 hens lay more eggs than 50 workers can eat. Just keep them and any chicks you choose to hatch later on in a place where there is no regular interaction with your workers, as chicks are very prone to become pets
An early thing i always do i bring dogs. Later on they cant compete at any level, not even as war dogs, but having a cage with 100 wardogs (ok, so only 20 so far) set to release as someone steps on a trigger way back will give you a good warning that something is up
later on you'll just use some trigger plates to get the enemy locked in trapped place, with battlements on the sides so that if they dont shoot, you shoot them, etc....
now i just got to make it like this