I used rather a lot of magma in
Rimslaughters (shameless plug). It's not quite what you guys are talking about, but it might give someone some ideas. Off the top of my head, there is:
1) A pseudo-pressurized magma cannon over the entrance (and a water cannon too). I haven't tried it out, because it will turn my moat to obsidian, but the water cannon was a little disappointing.
2) An semi-functional ice trap / back entrance which uses underneath magma to unfreeze the water. Still working out some kinks on that one.
3) An indoor lava moat, covered by retractable bridges, which swing back to force bad guys to walk along a 1-tile walkway full of stone traps, weapon traps, and ballista fire. Don't trip!
4) A layer of bridges above that walkway which can be retracted to drop baddies directly into the lava (or onto the walkway if they're lucky).
5) A series of magma spouts to fill that entire upper layer of bridges with magma, which is not only dangerous for its own sake, but which drops the magma straight down onto the walkway if the bridges are retracted...
6) A straightforward water/magma death trap which can be filled with water, magma, or both.
7) An arena floodable with magma (unfinished).
8) A chamber filled with booze, onto which 4 7/7 tiles of magma can be dropped, for explosive Fun (no luck capturing an imp or fire snake to use here).
9) A mini-magma moat underneath the bridge that enters the fort directly.
That might be it! But now that the aboveground is "reasonably" secure, I want to make an automated underground death trap which will probably use some kind of magma defense system. I have a ton of vertical cavern space to play around with (25 z-levels in places), so I'm thinking of suspended glass walkways with lots of different pressure plates. If anyone has a cool idea for a below-ground system let me know.