Minecraft XP Farm we build some time ago.
We wanted to have an enderman xp tower that has all the enderman in 1 place so you don't have to run along a long path like many other enderman towers do.
And we came up with this design for our vanilla survival server.
Many people fell in the void building this :)
* The tower is build over 250 blocks away from the main land, so that all enderman spawn in the tower.
* The water is there so enderman cant teleport anywhere. You need to have 35 blocks from the trap all water.
* The trap is made up out of 3 towers of 2x2 falls down with pistons on all sides where the enderman spawn. The vines are used to make sure the enderman don't get damage before the last drop to 1health.
* The added buttons increases the speed wich the enderman drop down, improving the rate of enderman at the bottom.
* at the bottom of these 3 towers the enderman are pushed together using pistons that are fired by a loop of redstone, from the bottom. (wich can be turned off).
* The last fall makes sure the enderman loose all but 1 health so you can kill them with 1 hit.
* And then we can just kill them quickly to gain loads of xp and enchant stuff and fix stuff on the anvil.