This one I came up with after about 5 minutes of hard thinking. You can make the trap short, to fit into a crowded and already extant fortress, or you can make it longer to fit more enemies at once for maximum gore.
Since it's very tight and not at all convenient for dwarves to pass through, I reccomend making this a second, false gate, connected into the fortress main with all doors to it locked. On the real gate, you can have a raising dr
awbridge/flooding moat/door/gate/whatever that is dormant until war, at which point you seal it off with a level pull and unlock the doors to your trap entrance.
| is the retracted bridge, + is the doors, and * will mark where the bridge, which raises --> thataway, should end when it's fully down.
The trap looks like this:
code:
################
#############++#
+*...........|.+
#############++.
###############
So when extended, the bridge will look like this:
code:
################
#############++#
+=============.+
#############++.
###############
And now: Pathfinding exploitation!
What you do is: Now that you've got your main entrance sealed, and the barbarians are at the gates, make sure the bridge is raised and unlock ALL the doors in your trap tunnel. You heard me. Now, since this is officially your only exit from the fortress, the enemies will come in here.
"But so will my dorfs!" you say. Yes! But if you have another door beyond this and keep it locked, you can keep dwarves away while still attracting stupid, stupid enemies. Now that you've got your enemies in the corridor, lock all the doors again and pull the lever you connected the bridge to. Splat!
To reuse, simply raise the bridge and then unlock all the doors.
It's a very elaborate and labor intensive setup, but if you put the effort in you could be able to defeat entire, dimwitted armies just by repeated use of this trap.
The fortress I tried this on worked fine with a full fortress guard, and standing army of 3 soldiers, and wardogs stationed in key positions against river/chasm men attacks.
Didn't help much against the demons though...