Over a number of games, I've never had anything worse than goblins attack the surface entrance to my fort. These are easily dealt with by a few doors and long lines of traps. I almost feel sorry for them.
However, I have had rather more...Fun creatures attack from below - specifically Forgotten Beasts from the caverns. Since these guys are building destroyers, my measly stone doors barely slowed them down.
How can I build a defense that will keep monsters this powerful at bay, or better yet, destroy (capture?) them outright? Are there different tactics available at a cavern entrance vs. a surface entrance?
I prefer constructed defenses over armies, but I realize that for some beasties, the only answer may be a platoon of well-equipped veterans.