i had a fake entrance hallway with a bait dog at the end, a few tiles before the dog there was a row of pressure-plates each of which did 2 things open the retracting bridges the floor was made out of and start a repeater, the repeater was connected to a bunch of spike traps on the floor of the chamber below the bridges. after a while i would pull a lever that de-activated the repeater(not gate connected to the floodgate that opens to start the repeater) and flooded the room with water flushing the contents into a rotting chamber bult over a bottomless pit walled with fortifications to let the water out. onve the corpses had rotted i lowered the bridge allowing access and took all the useful stuff and raised the bridge again. then i activated the retracting bridge making up the floor of the chamber permananty disposing of the crap.
only problem with this system is usually a few gobbos/elves/whatever got caught with the dog, killing it, so when i reset the bridges i had to kill the remanants and set up the pressure plates again and replace the dog
so all in all to reset i had to:
refill cistern
reset bridges
fight the triggering enemy
reset pressure plates
replace dog(or by preference cat/kitten)
it was all very inefficient but highly amusing.
note: i would have used magma, but didn't for 3 reasons. i only had a magma pool, i had no bauxite or adamantine and i am fairly sure magma can't wash things about. must test that sometime.