Because if it's not unreasonable, it's not dwarven.
I've been working on stage 1 in my current design, it's a wall of fortifications around the entire map, with a roof over it, and an inner wall to protect against anything that's 'dangerous' on the inside of my map.
The more complicated version counts out a space and puts TOWERS with BALISTA that don't, quite, reach eachother guarding the entrance. 75 spaces to the west and east of the entrance, firing towards it.
I can place this close to the edge, but not absolutely on it, infact I want to place it 2 steps BACK from the limit of where I can build. If I wall the entire map and put only one gap for trade to enter, then build a road to the edge of the world there, I've got a solid entrance.
Inside of this, would be traps, many traps, but phaze 2...
I'd LOVE to build a trade entrance that closes itself with a draw bridge that protects the trade depot from invaders. I'd love if that simultanously lowered another bridge that opened a 2nd path. IF this could ALSO trigger a repeater that would move, oh, a snaking hallway 1 wide, 10 long, that sweeps back and forth 3 or 4 times, filled with spike traps, as fast as was "useful" for making them automatically deadly...
Then if pulling the lever again stopped all the madness....
I think the lever has to go to a floodgate. The floodgate has to let water cover 3 plates that trigger on 3/3 water or higher. One toggles the bridge up, one toggles the 2nd bridge down, the third moves a gear to power the repeater? Then three more that trigger at 0/0 water? When the floodgate is closed again and the water is stopped (divert from river, drain into chasm, off edge of map?) the three plates that trigger on 0/0 water can then Lower the trade bridge, Raise the death bridge, remove the gear in the powertrain that feeds the repeater?
Because floodgates are slow to respond and water flow is slow, there's no way this process could get either of the bridges out of sync to the repeater, right?
THIRD STAGE: Pure INSANITY
Remember that wall, with the roof around my map? On the 2nd floor I build a retaining wall on the inside. Then I channel at the base of my wall against the world, the entire way. Then I pray that I've designed things so that this does not OOPS and collapse my entire map. I set up a few return paths under my wall back to the pumps to pump it back up to the top again. It spills beautifully the entire length of the wall on all 4 sides, down into the chasm.
What does?
MAGMA.
The ultimate defense, MAGMA MIST generated against the edge of my map.
Only trouble is immigrants are not as smart as caravans and will appear where the mist is, instead of on the safe road. Oh, and the insane FPS and POWER requirements... *cough* ... I did say stage 3 was insane.