Heh heh, glad to see you made it Puzzlemaker. Just remember that doors have no effect on demons, if you want to seal of the chamber effectively, build a wall. Walls cannot be destroyed by any means.
Once things have settled down to the point where you're reasonably sure nothing else is gonna pop out at you from the pits, you might wanna build a bridge or two over the entrance, just to make sure. Besides, it just looks cool.
Standard traps also do not affect demons. If you want to build a corridor to deal with the demons, you'll have to use spike pits or the like. I've never tried water, and haven't heard from anyone else who has, but it might be quite effective. Not sure.
Don't use drawbridge squishy-traps. They don't work, Toady's fixed that. If you try it now, the drawbridge will crack as soon as it hits a demon, and the demon won't be affected.
Frog demons aren't that impressive. Tentacle demons can give you a run for your money, but a well-trained well-equipped military can hack them to pieces (thus decorating your halls with hundreds of tentacle bits). Standard demons are rare, and can be quite a pain. Spirits of fire? Forget it. Batton down the hatches, and pray for underground rain to fizzle them.
That's why I was asking you about the engravings. A lot of times, the type of demon will show up in the engravings, so you can get an idea of what'll come out of the pits.