Early game creeper farm

Creeper farm that needs no mobs at all, no cats and no golems. A checkerboard of trapdoors on a solid platform gives creepers a space 2.7 blocks tall to spawn in and denies it to everything taller, and spiders cannot spawn on trapdoors either. The creepers walk into a central shaft, fall, and die on four campfires with hoppers underneath. It is the cheapest creeper farm here after the snow golem one and takes about twenty minutes to build. Java and Bedrock both work, Bedrock more slowly because it caps how often mobs spawn. It has to go over ocean or land, never over a river biome, where nothing spawns at all.

Do not build it over a river biome. Nothing spawns, and two of the most upvoted comments on the video are people who found that out after finishing the build.
Replace the campfires with slabs and kill with a Looting III sword if you want the rate. Builders report two to four times the output that way.
Add layers by copying the spawning platform upward. Three is the practical maximum, and one builder went from 288 to 960 gunpowder an hour doing it.
Break the line of sight in the kill chamber with slabs, trapdoors or carpet. Creepers that can see you standing there will explode on you.
Turn the render distance down. Two chunks concentrates the mob cap into the farm.
In 1.21.10 one builder had to go 120 blocks up over the ocean instead of the 100 in the video before anything spawned.