My embark is a plain with a high volcano in the NW corner, and my fortress is snuggled right beside the volcano. I've had two undead invasions, and they are a royal pain in the posterior, because the arrive on the volcano and won't path down to my fortress. I manage to trickle the first one off by building a staircase that they eventually managed to jump over to, typically one at a time, chasing my builder.
After that, I build an invader entrance tunnel where that force had appeared, but the next one appeared at the very top and won't move unless I sacrifice some dorfs to them (I've managed to get two of them by having them chase a civilian). OK, so I build a new zig-zagging (upwards) tunnel up to just beside them (and it was hell to get the miners to actually breach from the inside rather than trying to go through the horde). Yay! They chase my miner! Got two and closed the bridge. Get the cage traps reloaded and the captives hauled off. Open the bridge. No. A path into the fortress three tiles away? Who cares? OK, maybe the path is too long for their feeble brains? Shortcut a few bends (up/down staircases). No joy. They amble around in front of the entrance.
I'm sure I can get the buggers to enter if I provide some bait (as long as I can get the bait to run FROM the attackers rather than trying to path to the fortress' main entrance THROUGH them I should be fine), and I also think that will eventually take care of this force, but it will probably not be a viable work around against serious threats, such as Titans and werebeasts.
I'm considering digging a 3 tile wide trench along the edge of the mountain, and hope trees will grow on it to block the path, and further hope that will cause invaders to spawn elsewhere (feeble hope, though, the outpost liaison spawned between trees any my edge moat, so he couldn't get to the bridge. I had to cut some trees down to free the bugger).
A potential work around is to shear off the top of the volcano cone, as the attackers appear on the remote side of it compared to my fortress. The magma top level is at my fortress level, so I don't have a magma spill problem
Anyone have any advice on how to avoid attackers from just ambling around rather than attacking (and get summarily taken care of), as they ought to? If something is known about the deficiencies in their fortress path finding so those can be worked around it would certainly help.