I hope your not going into the third level caverns.... and better watch out, in our state a GCS or something of the ilk could probably take down the entire fortress.
QFT.
I'd suggest focusing on trapping all areas that connect the fort to the outside world and caverns, buffing our wealth for attracting migrants, building a pit for prisoners, forging some backup weaponry and trying to keep your head low in hopes that some rubes survive the trek to Bugger-all Nowhere, Population = us.
If you brick off an area for tree harvesting right away and make sure it has no leaks we should be safe from any FBs, but they like to show up at the most Fun times.