Don't know about you all, but I hate those mobs that wander up to your outer defensive wall and never leave. Especially the creepers (wonder why?), the spiders (they spidey-sense you through the wall and won't shut up), and the endermen (they're trying to steal the wall). I also hate wasting perfectly good arrows (and risking falling off next to a creeper) getting rid of them.
So why not build a wall that can do all that for you? Here's a prototype of the MobZapper 3000! Turn the mobs into mob-kebabs!
As soon as the mobs walk up to the wall, the pressure plates enable the Tesla coils. Each coil has a batbox (good for 4 shots), which is hooked up to solar panels. You probably only need about one solar panel for every other Tesla coil, unless you're dealing with a zombie apocalypse. They're all connected to one tin cable that's connected to the batboxes, so they get help from their neighbors. The red & white lights indicate whether the wall is armed or not.
Here's the view from under the outer wall. The wire on the ceiling on the left runs under all of the pressure plates. The one on the floor on the right is hooked up to the lockout switch.
Those two signals go through the AND gates and up the middle to power the blocks under the Tesla coils.
Here's the inside of the wall. The redwire in here is just controlling the lights. All of the Tesla control stuff is under the floor.