Necromancer fun: make adventurers, steal necromancy book, drop in your nearest castle and make a bunch of necromancers. Then watch them come to your fort.
Freezing traps: water freezes instantly when exposed to surface: build water holders just under the top soil and expose only trench wired with pressure traps in the middle, linking to doors holding back the water. Hallway of certain doom. Just send miners afterwards to clean up the solid ice.
Dodge traps: actually too easy to be interesting. Build two staircases: one on each side of your outside wall. Make them at least 15 z-levels tall. Connect them with a single tile wide pathway, 5-10 tiles long, and cover it in cheap weapon traps, making certain each trap contains the maximum number of weapons. The better at dodging the invaders are, the more likely they are to dodge off the edge and plummet to their death, and of course, if they fail to dodge, they eat the trap. This may or may not have changed with the new version.
Bolt recycling: After designating a narrow hallway for archery, dig pits where the walls were: when bolts fall a z-level after a miss, they don't break
Forgotten beast pitting: exploit an odd feature of cave-ins: the suction they produce to trap beasts without killing them. Design traps by carving small adjacent 5x5 chambers, each outfitted with a piece of furniture in the center tile, like an armor stand. On the z-level below this chain of chambers, dig out identically sized 3x3 rooms, after these are done, seal them, one by one, so they have no way out. On the z-level above the furniture, wire a single block wired to cave-in in the middle of the 5x5 and 3x3, using levered supports and sealing each prepared room similarly, to avoid flying F.B.s getting in that way. When the forgotten beast enters the room, it will stand two tiles away from the furniture, on the edge of the room and try to shatter it. This will give you ample time to pull the level and drop the block on the room. The block will destroy the furniture and blow a hole in the floor to the 3x3 chamber. Since the block isn't above the F.B. it will not kill it, however, the weird suction force created by the cave in in the sealed holding chamber will pull the F.B. down, into the sealed 3x3 room, from whence it will never leave (unless it can fly and its wings were not broken by the cave-in suction)
Puppy Jabberer bombs: fill a cage with jabberers and link it to a lever, beaked death will spill into the next goblin ambush, without giving the poor invaders a chance to ready themselves or shoot.
My own favorite fort technology combined the dwarven reactor with the waterfall. It was built into the ceiling of the royal throne room, which I made hollow and 7-z levels tall with engraved walls. The reactor was invisible, but the gears extended down into the royal pool, moving a single tile of water around the entire throne room and generating an obscene amount of happiness fog. Everything but the actual reactor was wired to collapse violently into the throne room, ideally both killing everyone near it and sucking everyone alive through a hole torn in the pool and into the untapped third cavern full of terrible forgotten beasts with deadly dust. The relevant lever was hidden behind the artifact bone throne of the necromancer duke.