Stackable creeper farm

Creeper farm that runs on a single cat and no redstone at all. Creepers spawn on stacked platforms, the cat sitting in the middle scares them off the edges, they fall down a shaft onto magma blocks that finish them off, and a hopper minecart under the floor collects the gunpowder. Nothing else is needed to make them move, which is why it is cheap and why extra layers cost almost nothing. Java only. Two things are part of the build and not optional extras: the AFK platform above it, and lighting every cave in range so the creepers spawn here rather than somewhere else.

The AFK platform is not optional. Almost every report of the farm doing nothing is someone who skipped it. In 1.18 builders use 96 scaffolding rather than 128.
Light every cave within about 50 blocks and cap the top of the farm with slabs or torches, otherwise the spawns go everywhere except into the farm.
Creepers that spawn in the far corners sit outside the cat's range, stop there and despawn. Replace those blocks with trapdoors, nothing would have fallen from them anyway.
Past five layers the creepers start landing on trapdoors instead of falling, and the gunpowder despawns up there. Pad the back wall with more trapdoors.
Build the spawning floors out of top half slabs and leave a row or two of solid blocks across each drop gap, or the creepers have nothing to pathfind towards and never step off.
One builder says that from 1.19 items no longer pass through the magma blocks and the collection has to change. Builders on 1.20.1 and 1.21.4 report it working as built, so check that before you commit.