Wondering if it's possible to make a gigantic floor-grate-covered room large enough for an entire siege of demons (with the sections not under a floorgrate blocked by a wall or simply not carved out) with magma and water pourable from overhead. I figure one then the other and we've got a pitfull of ex-demons.
Floor grates don't provide support, so it would be best to make your "room" more of a long hall.
the easiest way would seem to be this design... (cut-away view)
Z2 | Magma |
Z2 |Hatches|
Z1 | Grates|
Z0 \4/7 water /
Basically, demons enter and "exit" the hall on Z1. at either end, there's a ramp that forces them down into the main hall on z0, which is full of 4/7 water (dwarven bathtub). The "ceiling" of the hall is made up of grates on z1: building destroyers can't get targets on other z levels, so as long as they can't touch the grates where they enter, you should be fine. on Z2, the floor is hatch covers linked to your lever, and the z-level is filled with magma. When you pull the lever, the magma drops down, reacts with the water, and obsidian-casts everything in the hall. Very hot demons, though, might boil away the water: it would be safer to make the hall 'o doom filled with 2/7 magma or so, and the water be in the reservoir above, but that would be MUCH harder to build: dwarves can walk through the water to breach (and then flee from) the circus, while you'd have to have a separate access path for your miner if you fill the hall with magma... or just burrow your miner on the "wrong" side while it's dry, magma the lower hall, and then order the circus breached. still, more engineering that way.