Well, if you have a river above the entrance of your fort, and a chasm, you could have the water flow from the river through a pipe system, and fall in front of your fort and through grates and out a tunnel into the chasm, you could have one that doesn't require effort. Also, if spend enough time making it so that your dwarfs don't need to go outside for anything (with the possible exception of wood.) you could just get all your dwarfs inside during a siege, and turn on the defense system and go about on your daily life will all the invaders get mad and leave.
Also, lava would potentially be more likely to have a place above your fort that you could channel it through, but magma takes time to refill, so you'd have to use magma proof pumps in order to channel it back up and keep your defense active, but result would end with a bunch of free goods (and blood and body parts, can't forget those.)
