When I was a kid I got lucky enough to convince my parents to get me one of the sets that let you build a car, so it had gears and rods and pistons and shit. This is that set:
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It is a little hard to see the parts I'm talking about, but after building the car once my best friend and I proceeded to build tons of various fortifications and lined it with army men, then blew it apart with our lego ballistas. You'd take one of those long red pieces with the holes in it and wrap several rubber bands around it (covering the holes). Then you'd take the long black rod, like the one at the steering wheel, and put a piston piece on it to give it almost like a fist on the end. Push the rod into the piece with the rubber bands (only the back side rubber bands engaging though, the hole is too small to utilize the front bands) and then let go, and you could put a motherfucker's eye out. I know, because I almost did, and got in big trouble for it. However, when aimed correctly at the lego castles instead of my friend's eyeball, it would blast that shit apart after repeated shellings. Good times.