In answer to a request from Xenome, I made this tutorial on a simple cave spider trap. First, light the spawner. Gather your materials: some building blocks, torches, 2 iron bars, and some ladders, a bucket of water. Dig out the area around the spawner. Check your coordinates. In the directions that your X- and Z- coords increase, dig out 4 blocks from the spawner. In the direction that they decrease, dig out 3 blocks. Dig down under the spawner 12-13 blocks. Too little and the spiders will not take enough falling damage, too much and you will be too far away for them to spawn. Next, carve the floor into the shape here. Then place a water block on the highest point of the floor. In the corner towards which the water flows, dig out a small 2x2 space. Place the iron bars as shown. This is the killing square. You will stand past the iron bars. Make sure the water flows towards one point in the corner. I have a ladder on the side, this goes down to the item collection area. The item collection area is directly under the killing square. The items drop through the lower iron bars. And there you go.
raise the iron bars by 1 so they have a 1x1 box to climb into. they will all stack and stay in that 1x1. The way you have it setup atm i think theyll just keep pushing eachother from the killing area which isnt good if you want to use splash potions to kill.
