There, the obligatory spoiler warning is in place, and now we can refer to demons as what they really are. What I'm looking for are innovative, creative, or just effective ways of dealing with demons. I thought of one a while back which might be interesting to try out, and which could also be extended into a more complex version.
Essentially, it's a wide hall that allows the demons to spread out a bit. Above it, you channel around a hall-sized block of stone (for possibly more than just one level, for extra rock), and connect it to a support in the hall. The idea is that demons, although being immune to drawbridges and traps, are not immune to several hundred tonnes of falling rubble.
For bonus goodies, dig out the area beneath the hallway and seal it off. Now, when the cave-in comes, the demons are not only squished but also squashed as the plunge through the next floor. You can also, if you wish, fill this lower room with water (for spirits of fire) or magma (for the other guys. Try it sometime, you might be surprised). Or, just make it a really deep pit so that they get hurt by the fall.
This design can be used as a "last defense", which can be installed in a standard hall of death to deal with the possibility of the demons killing off all your warriors. Pull the emergency switch and BLAMMO! You now not only have wounded demons, you also have a sealed room containing said demons, since it is quite likely some of the falling rocks will block off the entryway. Plus, it's just damn cool.
Got an idea? Post it! Also, if anyone tries the idea illustrated above, please, please, please post a description of how it worked, some pictures, or a movie.