I've spent most of today trying to drop caged goblins/trolls in magma.
It's harder than I thought. First, I thought I could use a raising bridge to fling them into lava. Nope. Can't build on bridges, so I can't place them there. But I learned about how to build both retracting a raising bridges, so that was time well spent.
Then I channeled some holes above my magma chamber, and cover them in hatch covers, and create my pit. and assign them to the pit, and they instantly run away.
As do... deer... badgers.. racoons... really, anything not tame. Which defeats the purpose. If they're tame, the dwarfs will just drag them to the hole in the floor and shove them in. You don't even need to build the cages nearby.
Ok, but they're not tame! I've built walls on 7 sides of the pit, put single tile-width hallways leading to each hole, to force the dwarves behind the cages, so the victims have no where to run, and they still just jump out and run away, past the dwarf and out to chaotic freedom.
Is there no way to do this without burning two mechanisms per cage for every death? I mean... can't I just build the cages and then push them into the magma, somehow? I cage-catch a bajillion of these guys, I want an efficient means of disposal! (that isn't an atom smasher/falling bridge) I just want goblin-magma-puff-smoke. I don't care about the lost cages.
I've looked through the wiki and I get that you can use two mechanisms, and a lever to remotely open cages, but you apparently can't "transfer" goblins like you can tame animals, which again, is pointless because tame animals don't need special transferring! /grumble
Yes, I can just have the military there to turn them into bolt-porcupines, but the point of the cage traps (for me) is a nice packaging for disposal.