I have about 30 cage traps set up in two spots in my fort (around the entrance and in the caverns). I recently caught three goblin snatchers without getting any of my dwarves hurt. The goblins are now in the animal stockpile, along with a bajillion elk birds, a Gorlak, and a troll.
My question is: how do I get only the goblins into the Colosseum I am digging out? I intended to use these cage traps to get animals to tame for when my DM comes, not for fending off invaders.
Also, do I need mechanisms to open the cages? I'd prefer to have a fully armored axedwarf open it instead, so I don't have to go through the work of making 7 more mechanisms.
Cages can be built, same as beds or tables. They're 'j.' When you pick the one you want to build in a place you can hit 'x', which lets you pick a specific cage from a list that includes the name of the caghe's occupant. So your goblin cages will be listed as such.
The mechanisms aren't as big a problem as you'd think. When you're done you get them all back simply by unbuilding the level you pulled.
Before you do make sure you've disarmed the gobboes. With thieves it's not essential, but eventually you'll cage an ambush party, and it's quite satisfying to watch that legendary axegobbo run in circles as he's hacked to death by some of your crappiest troops.
To do that you need to dump all their stuff in a heal, and to accomplish that you have to unforbid all of it. Step one is to tell your dwatves where to dump gobboe crap. Create a zone with the i menu, and set is a garbage dump. Step two is unforbidding the crap. Use the d menu, hit b, hit c, and then select an area around the gobbo cages. Now you tell the dwarves to dump the stuff with d-b-d. IOf you've done it right you'll have dozens of "dump item" jobs on you jobs screen.
BTW, when you get advanced you can decide to only dump specific things. Arena fights last a lot longer, for example, if you only dump their weapons.