Minecraft wiki entry for Spawner_traps, associated text: An alternative layout with a smaller footprint takes advantage of the fact that a player must be within 16 blocks of a spawner for it to activate, but that spawners only check 9 blocks vertically for spawned mobs. Build the trap as above, except instead of being off to the side, the killing floor is 4-16 squares directly above the monster spawner. This requires less material, less x,z space and less precision in construction, though is 3-15 blocks taller.