This is a trap making use of a vertical staircase spawner (same as used in my drop trap here: www.planetminecraft.com/project/flyingpenguin33s-mob-drop-trap/

This trap uses 18 vertical spawning towers, each with a drowner designed by Brewtalizer on it's output. The drowner makes use of the fact that water moves faster adjacent to glass versus other blocks. Mobs fall and float in the air pocket near the bottom which keeps their heads in the block of water above the air pocket, causing them to drown. And yes, it also drowns spiders (VERY slowly).

This trap was built to maximize item production in a relatively small footprint. It produces 1800 items per hour (when only one player is around), but is less than half the size of other traps that produce that quantity.

The schematic I made available for download was copied from the upper right corner facing west. You should be at an altitude of 126 above bedrock (2 blocks from the top of the world) when pasting. Assuming you paste it according to my instructions, the bottom of this trap will be 50 blocks above bedrock. I would recommend filling any caverns and voids underneath and immediately around the trap, and lighting up the nearby countryside to maximize the number of mobs that spawn in the trap.

Additional Notes