Boat clock mob farm

A cheap mob tower by LaZ1en Farms with a boat riding a water loop as the clock, which periodically opens the trapdoors holding water back on each spawning floor and washes everything down into campfires. No redstone beyond a short clock, and the whole build is around fifty redstone, forty slabs and a handful of trapdoors and signs. Water at the top of the drop drowns spiders, which climb the walls instead of falling, and the same flow pushes drops into the hoppers.

• Most reports of it producing nothing come down to the AFK spot. Mobs only spawn within 128 blocks of a player and despawn past it, so the platform belongs directly above the middle of the farm at roughly 127 blocks from the drop.
• Light everything within about 50 blocks in every direction including the caves underneath. Builders who skip that get almost nothing because the spawns happen elsewhere.
• Trapdoors do not hold water back on Bedrock, which is the one thing making this Java only. Building it over an ocean instead removes most of the spawn proofing work.
• The water at the top of the drop is there to drown spiders, and the same flow carries the items into the hoppers, which LaZ1en explains when asked.
• To take XP off it as well, put the slabs below rather than above so you can reach the mobs, or swap the campfires for slabs and leave a half block gap to one shot them.
• Trapdoors on the ceiling of each level turn it into a creeper and spider only farm.