In addition, obsidian-cast 3x3 prison rooms, high above the main entrance and suspended by a support on top. When a siege comes by, I'll drop the previous siege's members on them, plus large amounts of stone. I might convert some parts to nobles' rooms, so that unfortunate accidents may be deployed in this manner as well.
Sounds good. Trying to remember if I know what happens if you obsidianise a (magma-proof) cage, though, and what happens to the creature within it...
Because I was thinking of a 3x3x3. Cast a 3x3, place cage of enemy on centre of top.
Either carefully cast the 8 other tiles of that layer or cast the entire layer. Then a final 3x3 on top (being careful with the centre one if trying to maintain the cage-containing void).
And even if the casting on a built (or free-standing?) cage causes severe life-limiting conditions to the caged
noblecreature, it would remain symbolic, though, and it might even lead to (less effective, but more symbolic) stone-fall traps loaded with the obsidian that was cast around a cage enclosing the (remains of? living body of?) previously captured.
If not walls, floors... fortifications?
Oooh, oooh! Turn the blocks into mechanisms with which you can build your next cage-traps, providing the 'materiel' through which the cycle repeats! "The road to hell is paved with... strangely familiar-looking fossilised remains. Hey, is that the famous general who was last seen leading troops here in a previous attempt to destroy this dwarven dump? Sorry, mate, was that your nose that I somehow managed to depress? <click> <Whoosh!> <clank> Gargh!"