This explanation turned into a bit of a story-like ramble, so spoiler-tagged.
The worst thing I ever did I never completely finished (though came damn near close to) - I set it up on my first fort, actually, after the kids started becoming adults and sieges had gotten monotonous.
As many first fortresses do, my defense consisted largely of traps. In this case, I had set up lines of cage traps between archer towers, forming a lovely perimeter. This led to, over the years, about a hundred and sixty-odd goblins being kept in storage, including all the local leaders. Eventually I decided to do something with them. This led to the Goblin Games room.
Upon release, goblin POWs pathed as best they could towards the exit. This took them into a 5z level labyrinth, about 25xy wide. Rather than simply going up, it went up and down and all around a few times. Rather than simply running in a straight line, I channeled out holes covered by bridges and set up three different repeaters to attach them to. Pathfinding would not have a good day here.
The second room was a simply path over a magma pit. However, archers were stationed on both sides of the pit, starting from a good distance away and finishing within a tile from the running goblins. Avoiding the boltfire was close to impossible.
The goblins were then herded into the arena. At this point I would lock the exit door, and all the goblins who made it through would run into it, realize they couldn't escape, and stand around idly. After a fair few were through, I released the Giant Eagles. The elves brought me a breeding pair, you see.
The survivors, rare as they were, got to run through another bane-of-pathfinding room. Ten thin but long platforms were littered with spike traps, each platform being connected by three seperate bridges to the next. All the spikes and bridges were connected to different repeaters, forcing the goblins to take different paths and run over the suddenly-protruding spikes. Those who fell simply limped back under the platforms to the entrance of the room and tried it again.
Few made it through those four rooms. However, I did have some local leaders, and I like the number five, so I thought a fifth room would be nice. The one or two goblins that made it through the hell I laid out for them found themselves in a nice, tall, cone-shaped room. The wide circular base tapered as it continued 15z up, with a thin path following along the walls all the way to the exit. Upon escaping the cone, if they could contend with the single giant eagle waiting at the exit, the goblins would be free to go. However, to begin the ascent, they had to step on a pressure plate.
Next to the cone was a resevoir containing 3000 blocks of 7/7 water.
Think Wind Waker, just before the Helmaroc boss. The goblins ran from the water, which rose insanely fast, while an eagle persued them around the room. Even if the eagle didn't shred them apart, they were likely to drown.
I mentioned earlier that I didn't manage to complete this hell. The labyrinth, marksdwarf passage, arena, and flooding cone were all done except for the bridge room, since I tired of assigning spike traps and bridges to connect to the repeaters after the first ten or so. This didn't mean I didn't get to test it. It took the gobbos about five minutes for the first few to get out of the labyrinth, after which a surprising amount made it through the marksdwarf passage (they were just recruited peasants; I figured enough bolts would be in the air that it wouldn't matter), with one memorable gob falling off the pathway into the magma pit. The arena went pretty well with both giant eagles killing about eighteen goblins together before being ripped apart, and the few spike traps I had working actually managed to hurt some of the survivors. The last five goblins made it into the flooding cone (the pressure plate locks the door behind them, sealing the other survivors in the spike room), which I was the most excited about. As they ran/swam for their lives, the two slowest ones drowned, the eagle killed one and tossed another down into the center before drowning itself, and the fifth - a local leader Axelord who was Unbelievably Agile - actually made it out.
As he hobbled towards the exit, wounded and traumatized (some yellow wounds and a missing hand), I wondered what kind of life he would go back to in his home tower. He had been missing for nearly a decade. Would his family have moved? Died? What kind of life could a goblin lead bearing the guilt of being the sole survivor of such sadistic carnage? Surely the poor fellow would be traumatized for years to come. Only the sight of sweet, sweet edge-of-the-map freedom was keeping him going now.
As he got five squares away from the map's edge, I sicced the other ten giant eagles on him.
And that, my friends, is how Dwarf Fortress has made me a bad person.
Might come back with one or two pics later for illustration.