My plan is to carve out a long, meandering tunnel filled with 2/7 tiles of water. When I'm under siege, I open the tunnel entrance allowing the enemy to pathfind to my meeting halls and waltz right in. Once they're all inside, I shut the gate, sealing them in. Further down the winding tunnel, they quickly discover there is a weapon trap in every single tile, each armed with a low quality whip made of copper or bronze. (Since dwarfs don't craft whips, this clearly took years of trading and forethought.) The light gore damage isn't enough to kill, but it will wear down their health. Slowly flesh is shredded away, ears ripped free, noses sliced off, eyes gouged out, fingers and toes messily amputated. The lucky ones die from pierced organs or blood loss, but not many are lucky. Morale is now broken, and they want to flee, but the only way is out to continue forward down the long long hall. The mutilated victims' heavy bleeding begins to stain the water and walls a crimson red. They're crawling now, and vomiting from the pain. They pass out every few feet, making the last leg of this tortuous journey seem a hundred times longer than the hell that preceded it. Finally, they emerge from the dark, watery nightmare. A wide plaza lies before them, constructed of mica blocks glistening in the sunlight. They drag a bloody path across the beautiful white plaza. The edge of the map-- and freedom!-- lie so close. They're so very close to escape, and then...
Then I let them go. I let them go to live a life of abject misery in their homelands, spoon-fed by their lamenting kinsfolk and shunned as hideous monsters by their callous neighbors. For years they stumble on in a state of near-constant agony, punctuated only by intermittent episodes of unconsciousness which bring dreams of that long long hallway. One night a single guard is found dead at the edge of town. Further investigation reveals the bodies of each of the crippled veterans of that ill-fated siege, all lying dead in their homes and bristling with adamantine crossbow bolts. A lone assassin had slipped into town to punish them for their bold trespasses on dwarven ground. The message is clear: No one gets out alive.
Now that's a trap!
(sorry got a little uh... got a little carried away there.)