I read somewhere on these forums that retracting bridges used over solid floor will throw creatures without blocking their pathfinding. May not be true now, but if it is...
My idea for a gravity trap is to force goblins to travel in alternating directions along z-stacked single-width corridors covered with retracting bridges.
From side-
/ or \ = ramp or stair
_ = single-width floor with retracting bridge over it
\____________
____________/
\____________
etc as far down as needed
Most importantly, there is open space immediately to the side of all bridges
From the top-
O = open space
F = floor with retracting bridge over it, as far as desired
w = wall
R = Up *OR* down Ramp (or stair)
OOOOOOOOOOOOO
WRFFFFFFFFFFFRW
OOOOOOOOOOOOO
...stacked on top of each other for many z-levels until last level (killing floor) is just plain floor with no open space.
The retracting bridges are attached to a fluid repeater. For extra convenience, attach a 'master' lever to control both the repeater and entry to the system. Only one lever thus controls the whole system.
Naturally, the goblins are thrown off the bridges and fall down the sides. Upright spear traps (which can't be dodged if fallen upon when in the upright position) are placed in the areas the goblins will land. To increase lethality add more z-levels to the fall from the last bridge traps to the kiling floor, ensuring walking access from bridges to killing floor remains. To bring the goblins in, place bait on the killing floor. This system should work on creatures with the trapavoid tag.
It'd also be wise to place weapon/cage traps in the last corridor to deal with possible survivors who were quick enough from ramp to ramp. Survivors are less likely with longer corridors and quicker repeaters. Also, the walls can be fortified to allow marksdwarves to fire in. Ballistas would work great in these long corridors and help deal with the megabeast problem.
Bridges won't retract if megabeasts are on top, so use building-bait branched off before the first bridge to entice them to walk over cage/weapon traps rather than the bridges. This will be less effective if building destroyers don't attack the nearest building first. To deal with flying creatures coming in, place the statue/creature bait in a corridor off from the killing floor and stuff the corridor with weapon/cage traps. If there are flying, building-destroyers creatures with the trapavoid tag, then you're pretty much screwed anyway...aren't you?
For extra convenience, use magma to clean the killing floor while killing anyone who reaches the bait on the bottom level. Make sure the lever that lets magma in will close the route back upwards. Place killing floor near magma furnaces, perhaps on same z-level.
I'm also toying with another idea of letting goblins walk down a very long up/down stairwell, and manually flushing water downwards from a cistern at the top to force them to fall the whole way down and land in a spike pit.
I have some questions-
How long will it take for a retracting bridge to respond to a pressure plate?
How far will a goblin have to fall for his body to disintegrate? How far to disintegrate his weapons?
Will building destroyers attack the nearest building first?