Quote fromMobs won't spawn on a tile covered by water. Remove the last two sets of half-blocks and water, then carve out another 10-20 blocks past the back wall. You should also build a catwalk above the trap that extends into the dark zone for kiting purposes.
Quote fromMobs cannot spawn in water. Take out the last 2 water conveyers and just have cobblestone.
Oh thanks! I got it to work.