I'm personally debating the logistics of an auto-drowner + mist generator. In theory, my fort will have a large water reservoir atop it, some 3, 5, or 7 Z's high and probably 19x19 since my footprint is usually 21x21 and it needs walls. Water is pumped in from the nearby river, through an above-ground aqueduct, and into the reservoir, where short pump stacks inside the reservoir itself make sure it's filled. This is done by placing two pumps atop each other and then a windmill atop them, like so:
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Placing this inside the water tower, I only need my aqueduct to reach the floor of the reservoir, and then the internal pumps will pull water up one level, which will create a void on the level below which the aqueduct will instantly refill, thus allowing a somewhat slow but very easy setup and low maintenance filler.
The front of my fort will consist of a construct bridge, made from floor and not actual bridge, with pits on either side. Normally, the reservoir will fill to capacity and then let the water out through the top, which insures that the reservoir will always be full before it lets off the excess water into the out-pour. This normal out-pour courses through the fortress creating a mist generator, dorfbath, hospital water source, hydro reactor, etc.
However, at the onset of an attack, preferably trigger by a pressure plate on the narrow bridge, the reservoir will instantly open several hatches on its lowest level, letting every Z level of water above it rush out (possibly pressurized and/or pressure nullified for perfection) which will push any invaders off the bridge and drown/crush/impale them in the pit below. Once killed, the waterlogged corpses and loot will be pushed by the flowing water, over a horizontal floor grate drain which should collect all the loot in one convenient location inside my defenses.
The activation of this trap will shut off my water, and the mist generators and running flow will cease. Once the invaders are defeated, the hatches should automatically close, the reservoir will refill, and the ecstatic-inducing flow of water will resume. This feature can, and will, be used as a FB defense mechanism as well, by letting the water drain down into the caverns. It will create a soothing bath for any soldiers, cleansing them of any FB toxins as they return. As a safety measure, if the water is not running, a pressure plate will detect the lack of water and will shut the door, thus only allowing soldiers to return from the cavern if they're properly cleaned first. The same will likely be true of the main gate as well, because dwarves won't wipe their feet unless it's done violently, and I'll probably make some measures to wash through my butcher area as well, since I've observed butchers to sometimes create blood on the ground. Besides, it'll just be cool to have a butcher area with tile floors and drain outlets!