Ninety cat creeper farm

Creeper farm built as ten stacked layers with cats in the wings, no golems and no redstone. Creepers spawn on trapdoor checkerboard platforms, cats push them toward the middle, and water carries them down a central drop onto four slabs where you kill them by hand. Trapdoors go on the ceiling as well as the floor, in the inverse pattern, and that is what keeps zombies and skeletons out. It is the most expensive creeper farm here by a wide margin: 146 stacks of blocks, 110 stacks of trapdoors and 90 cats. Java only. Swap the four slabs for campfires if you want it fully AFK instead.

Trapdoors go on the ceiling as well as the floor, in the pattern opposite to the floor. The video does not show this clearly, and it is why people get zombies and skeletons instead of creepers.
Replace the four slabs over the hoppers with campfires if you want it to run unattended.
Use a Looting III sword rather than the campfires if you want the headline rate. The campfire version is roughly a third of it.
You do not need 90 cats. One builder used three and still filled two chests overnight, and another says two floors with no cats at all works fine.
Do not build it so high that the water freezes into ice.
Ten layers is the full build. One builder at ten floors with nine cats each reports a double chest of gunpowder every twenty minutes.