The minimal minecart grinder works adequately: we were visited by a siege of 80 units, but they quickly decided it wasn't worth continuing after twenty of them ended up somewhat flat. A few stragglers brought the count to 26. Interestingly, sieges break and turn tail fairly quickly when they can see the corpses of their comrades, but won't do so if there are no corpses (i.e. atomsmashed).
I hadn't really thought of the thing as a minecart grinder, but some posts mentioned it and yes, it works as a grinder. It's not very flashy, though. I built a design with several of them chained up and some machinery in place to work as a reliable off-switch, and of course the trolls went and blocked the first cart while they took a hatch apart. Once they got moving again, they spread out and were quickly annihilated. Nonetheless, empty silver carts have trouble when trolls decide to clump together.
And then there was the annoying winged monitor lizard that was camping in the third cavern level: it was completely stuck in place (in the air) for years. It was the only beast that was still alive, simply because it wouldn't path inside.
So we built a four-level pump stack from the magma sea directly below the cavern floor, carved a spout into the wall two levels above it and let the magma flow. Took a bit of fiddling around with an extra spout to actually hit it and we created two sizable magma pools, but it patiently waited until it was doused in magma, caught fire and died. Power comes from an aquifer channel across a gradient, sent down to the bottom via a 60z vertical power trunk built a few years ago just for the hell of it.
This is a no-military tinkering fort (started with "how to punch through an aquifer with a cave-in"); current score: 957 hostile dead (forgotten beasts, kobolds, megabeasts, goblins and the sieges we got after spiking the human demonic lawgiver), 186 non-hostile, of which less than ten were citizens, about half of which died to hostile action, half to accidents.