Minecraft wiki entry for Spawner_traps, associated text: To construct the trap this way, build a 9×5×9 room around the monster spawner with flowing water leading the mobs into a corner. Build a water ladder in this corner going 26-38 blocks upwards as measured from the top of the monster spawner. At the top of the ladder, direct the mobs a couple blocks in any direction with water, and into a 22 block drop chute. At the bottom of the drop chute, create a platform 1 block lower, with a 1x1 "window" into the chute through which you can strike the mobs safely from the platform.