I built a goblin abattoir once, mainly to see if it would work. It was very complicated and I don't have pics but I'll try to explain.
I had two entrances into the fort. One main entrance that could be sealed off with a drawbridge with a moat in front. The other led through the abattoir. I used hatches and pressure plates to make it a one-way entrance. (See the goblin grinder:
http://www.bay12forums.com/smf/index.php?topic=62798.0) Then I added some spike traps and drawbridges that opened above a long fall with pointy spikes underneath to prevent the hatches/mechanisms from being wrecked by building destroyers. After the one-way entrance was a long, winding hallway leading into the abattoir proper. This was designed so that the entire siege would have a chance to enter before hitting the traps and panicking the rest of the siege. The abattoir itself was nothing more than a large room filled wall-to-wall with weapon and cage traps.
A nice thing about gobbo sieges is that once enough of them are killed, the rest panic and try to bug out. After enough gobbos got trapped/ground into chunks, the abattoir would be full of panicking gobbos. That's when I'd open the side door and the military went in to mop up. The gobbos are too panicked to fight back, but still armored, making it some slight challenge for the military to kill them, getting a leetle bit more xp in fighting skills than they would against stripped opponents. Very quickly they became legendary. Once everything was dead, the civvies went in to collect the goblinite.
Once a siege entered, nothing ever escaped.
I eventually stopped building goblin grinder-style abattoirs because it was just too easy.