I don't really mass pit, as I use my prisoners to train military dwarves.
How do you get them out of the cages without exposing a hauler to being attacked? I'm trying to avoid the whole "build a cage, link to lever, pull" system.
But anyway, wouldn't it possible to channel a 3x3, then just reconstruct the perimeter floor, with the hatch in middle of the 3x3.
It will mean there's a 1 tile buffer all-around the hatch immediately below. So they can't grab on to anything, as there is nothing to grab.
That is just the wiki's mass-pitting system with a single hatch instead of multiple hatches, and even with more space below, the prisoners seem to start fighting instead of falling in, although I now am wondering if raising the ceiling above the cages would help.
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I usually have multiple rooms with doors, build cages ( up to 100 a time) link them all To a lever, lock the doors and pull. Unlock the doors one at a time to have my military take on anywhere from 1 to 9 prisoners. The rooms are usually 3x3, but I've done 1x1. I havent mass pitted since climbing started because I don't like goblins running amok, armed or not.
Alternatively have your arena, above that have your pitting room. Your pitting room should be sealed except for your pits, and gave your vampire and/or were beast live in the pittihg room. Have a minecart or similarly over engineering contraption deliver cages to the pitting room.
That would work too but is much more effort, but that way you have no worry of goblins escaping and harming someone important.
Or, much simpler, set ten dwarves maximum to do pitting. Set them to a military squad, make sure to keep it inactive and have uniformed while inactive set true. Equip them with leather armor and obsidian short swords, that should be plenty to kill a unarmed goblin. Have weapons and armor for your guest at the bottom of the pit if desired. That way anyone escapes your haulers are technically militia and will just kill the poor saps.