I've got my civ ethics such that goblins are butchered. Dumping them in magma means no -goblin tallow roasts-.
Goblins are dirty. They live in the mud and they shorn trools for wool. Go eat some pigs, elephants, or sharks. The goblin soap isn't a bad idead, though.
You could fill it with magma and stop worriyng about how deep it has to be. You will not get any loot if you don't have any drainage system too, but at least the invaders will die a horribly melting death.
that's dwarfy, i suppose. but definitely not amusing, not at all!
seeing gobbos trying to get its way in/out of the fort with severe injuries or even missing limbs, while spittig its last litres of blood all over the place is fun!
and loot could be good. goblinite, useless clothing and leather armour for sale or for magma.
well, since you mentioned magma pit, i think a combo i.e. one side magma one side deep pit would be hilarious.
a harsh choice for you foe, between a very quick death, or slight yet painful chance of survival.
Yes, that or you could make an, let's say, 8 Z deep pit with menancing iron spikes in it, so the goblin's and trolls fall and suffer horribly injuries, but don't die. Not all of them, at least. Them you put some dogs to kill the rest and feed the dogs at the same time, fill it all with water and before everyone drowns, flood with magma. You will gain obsidian, goblinite, insant bones and you could make some nice obsidian scultures and statues of goblins dying, and your engravers you probably engrave on your halls about the "enclosure on cooling magma" of your enemies.
It's a shame your fort can't gain fame. Imagine the goblins thinking twice before laying siege to the goblin eater dwarfs of Hyndis.