I just invented an über leet defense system for my fortress, which is really cheap to make and even cheaper if you happen to have a brook / river / stream at hand. I have a 3 tile wide corridor which then splits into four 1x1 corridors which have a pressure plate with floor hatches on both sides of the plate, which are opened by the plate. When an intruder steps on the pressure plate, the hatches open, forcing the intruder to stand still on the plate, then after that a hatch which is also linked on the pressure plate opens above the intruder, dropping water on the intruder, which then pushes him into either one of the two hatches next to the pressure plate. The pressure plate is then deactivated due to water pushing the intruder off the plate, closing the 3 hatches and while the intruder falls into a 15 z-levels deep pit which has grates at the bottom that let the water drop into a cavern below.
I was thinking along the same lines, but for capturing FBs and clowns into cages. (My Clown Zoo needs them badly.)
I have no idea if this would work, however. Haven't tried it yet.
Anyway, my idea was: a long narrow walk up beside a fall of few z's, with cages and drain at bottom. Said room requiring the kicking in of a floodgate to gain access, (with animals in leashes to pull 'em in,) And with 7/7 water between doors to flow onto a plate to trigger additional hatches to open up from above and let a tank of water flow in with small delay and pressure that would hopefully throw some clowns off balance and cause them to fall to cage traps below. Maybe with replacing said tank of water with a cave-in option, if just pressurized water doesn't do the trick for the clowns. I dunno yet. Must test, in due time. Interested in other traps that people have used to cage fun stuff.
I mean, I can't really colonize Hell without some demons to show people in a Zoo, can I?