Just thought of something. You'll want to have some kind of backup at the exit since I'm pretty sure the hatches and pressure plates could be destroyed by Building Destroyers, leaving it a straight path into your fort.
This never happens! If they break the hatches, they trap themselves because they cannot path over the channel. I had a forgotten beast lock itself out like this. It hit the plate, opening the hatch, got mad and smashed the hatch because it was "in the way".
edit: I wonder if there's some way to allow dwarves to get through without anyone else allowed. I can't think of one though.
Citizens trigger is separate to Creatures trigger, so friendly units, including traders, can pass these things fine, but wild animals and invaders cannot. So you can do this already, as long as you are happy with the games definiition of "dwarves" and "anyone else"
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I used HP (hatch next to pressure plate) to lure/herd hostile creatures, and HPH to immobilize them on a single spot. It's how my drowning trap works. The Plate is also linked to a river water hatch straight above their head, and goblins invariably (and i mean 100% of the time for about 50 'test' goblins) get washed into the channels. There's a bit of backup insurance in case anything doesn't get washed away: the entrance to the tube is a down ramp from above, and on the exit side, the plate raises a drawbridge, so it'll drown or isolate anything which stays on the plate. So far, no problems, but there's cage traps past the bridge just in case.
This (HP combo) is also useful for designing GCS silk farms.
The drowning tube worked great for me, with a bank of parallel tubes so there's always a path, and they reset while the goblin below is still drowning, so they don't lock up as long as the goblin grinder. There's also a floor grate just past the bridge to speed drainage.
Only problem is lots of goblins give up and run away when a number of them are killed. I'm dusting off an older design to solve this issue, which is a room with many HP exits which keep goblins in the room, and seems pathable to inside the base, but further in is a HP that keeps citizens out of this area. The idea is to herd entire sieges to path at max. rate into this area, always thinking they can path further in and are winning. When it's too late for them to run away, the area will be isolated and a number of execution methods can be manually triggered, including firing squad.
At the "main" entrance for my base I'd have more HP switches to keep gobbos out as a failsafe, and a drawbridge. Raising the drawbridge would make the only path inside be the trap room.
Whenever using these to keep out nasties, make sure the channelled tile under the hatches do not open into your fortress or you could have some problems.
I always worry that one of my citizens will be chased by a goblin and end up falling into the drown trap water or a hole in the ground. It hasn't happened yet, but it's possible. I do have a "drain" lever for the drowning chambers though, so could rescue any drowning dwarves this way.
Everyone so far seems focused on the enemy grinding systems, but I think that the high-speed trigger part of this has a lot of potential.
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Can this design be made a bit more compact i.e :-
DH^D^HD
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DH
Horizontal paths are technically shorter. Going by the path-finding threads, I'd assume there's less time needed to move horizontally.
Better still :-
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This design would have two different trigger events on the bottom two plate/hatches. Forbid the top (shared) door and open both bottom doors for the creature to cycle the two actions constantly. There could also be other actions linked to the top two hatches for 'idle mode'. If the shared door was separated again, then you could basically have 4 different repeaters, and guarantee at least any two of them can be active at once.