I believe for a Dumping Zone to work over Z levels, you need to place the zone on the walkway around the Z drop. Something like this:
Code:
+++++
+ZZZ+
+Z.Z+
+ZZZ+
+++++
Where...
+ is floor
Z is your Dump Zone
and . is the open space leading to a vertical drop.
I've had success with the Pit/Pond zoning method, by tunneling down to the magma layers to throw POWs down. My recommendation is this:
- Minimize walking distance between cages and execution hole
- Lay cage traps down along the path to the hole, as well as any area leading into the fort
- Order a prisoner to be thrown down ONE AT A TIME
- Repeat
Every now and then, I get a "Break Out" where the prisoner isn't handled correctly and is freed instead of being hauled to the hole, but if you anticipated that with traps, then they should be recaptured so you can try again very shortly. That's a pretty reliable method for getting rid of POWs in my experience. Lets you reuse the cage when you're all done, too.