So I've been building a mighty zombie pit for fortress defense. All undesirables are tossed into the zombie pit. Its not the fall that kills them, its the hundreds of endlessly reanimating zombies.
This leads to a massive accumulation of zombies, partial zombies, and precious scrap metal at the bottom of the pit. Getting the undesirables into the pit is trivially easy. Cleaning out the entire pit is also trivial.
But I don't want to clean out the entire pit.
What I want to do is:
A) Extract valuable iron from the pit.
B) Do NOT destroy the zombies, they need to be recycled for the next sieges.
C) Do not let the zombies escape the pit.
I considered using a combination of bridges and water, to cause creatures to flee to dry ground if possible. The iron would sink to the bottom, and I can use bridges to cover the top of the pool while dropping out the bottom of the pool. This works fine for living creatures, but zombies just don't care because they don't need to breath.
While I can just brute force it and barge in with my military to murder all of the zombies, this inevitably leads to a massive mess, as hauling away the iron is continually interrupted by zombies, and zombies end up leaking out of the containment pit, body parts, panicking dwarves, and legendary soldiers with axes chopping zombies everywhere. I've tried this, and it causes the entire fortress to grind to a halt and descend into mayhem. Its not dangerous, its just not productive at all.
Slaying all zombies with spike traps does work, but they reanimate so fast no hauling can be done. If the spikes are not continually moving, there will be zombies up. If the spikes are moving, haulers cannot enter.
Any thoughts on anything I've missed that may allow for the sifting of zombies, keeping the zombies contained and safe, while allowing the valuable metal to be recovered?