I've been bumping back the job of laying down the grate that would open the path to my fortress, but it manage to slip by me while I was deepening trenches and picking at quality veins.
Holy clusterfuck! Just when the pitfall path opened, my best miners and fighters have trapped themselves underground, and a beast shows up underground. I deployed B-squad expecting them to get slaughtered, and another FB appears. Someone finally pulls the lever to open the inner lock so my miners don't have to carve a new path to obviate it, and when I look back to my squad, a farmer is carrying off the corpse of the first FB. None of them had any injuries, and after checking their weapons I still can't see who got the kill. It turns out the second FB is in a cavern still sealed with a grate, so won't be an issue for awhile.
The invaders continued their march. And while I watched them sail off the cliff, I learned a few things. First, that trolls will destroy the path to their destination. Good job asshole, now no one is getting through. Second, that a caged enemy can block the space it's trapped in, so none of it's buddies will be walking through, and 8 goblins get to hang out on a ledge. I learned that goblins chucked off 4 z-levels still have enough fight to fuck up a war dog, but not quite enough to get past a war gorilla.
All the time and effort on these traps took out almost a 1/4th of the invaders. Not a bad deal, except that the surviving crossbowmen are parked at a vantage point over my courtyard and over the traps that I should be shoring up. They should have the ability and AI to advance, but they aren't, and that bugs me. But even if I clear them out, there are still trolls sitting in front of a gap that should've been a walkable pathway, and no civilian is going to restore that.