Yes, the thought of the moat and cart combo came from a desire to combine sciences, but imagine our already crazy amount of ways to kill something multiplied by Minecarts.
Giant spike launchers, anyone? No more relying on catapult siege operators that hopefully don't get scared off, pull a lever in the dining room and watch as invaders get impaled upon death spikes.
(That's Class I Science, using one mechanism with semicomplicated engineering)
Flaming or Melting giant spike launchers:
Class II science, adding a lava-fill or a flaming granite track/loading bay. Note the need for reignition and re-graniteification, I think.
Fists of Justice
(Not sure if Class II or Class II, note that classes are just the amount of different sciencey mechanisms in one place... This oune might could spray lava too...)
This one is like the old landmine traps but automated, and when set up, it's a sunroof for your dwarves. Make a minecart rolling down a track trigger some supports holding up a bunch of floor tiles over the most likely siege path holdup- like in front of your fort.
Pull lever, minecart begins it's journey of manliness.
Minecart reaches a station very close to a "port", a floodgate blocking an entrance where a rail is, activates floodgate
Minecart passes through gate then deactivates it
Minecart goes across some detectors, setting off the mines and crushing some goblins
Minecart does the enter/exit floodgates to get back in from a different port.
Minecart completes it's journey of manliness.
You could also set different levers to different mines, and have them much higher than your classic landmines. You could also hypothetically chain someone to the collapsing roof. Endless possibilities.
You could also have minecarts that activate other minecarts. This could mean putting a detector at the end of your rail and setting them up to fire five times for one lever pull. Fill with coins for the lulz!