Finally, after 12 years of tinkering, my ridiculously complex, dangerous, and thoroughly Dwarven clockwork Drowning Entryway manages to intercept a goblin ambush (led by a snatched elite marksdwarf of all things) and drown the invaders. Sadly, two of the gobbos are pushed into cage traps by the water, leaving a third ambusher to drown and then fall through the draining hatches.
Definitely not the most efficient design out there, but at least it worked dammit.
Also, big shout out to Markavian for his CMV editor. This video would otherwise have been interrupted by a caravan arrival.