Minecraft wiki entry for Creeper_farming, associated text: When trapdoors are placed on a 2 block high ceiling, it lowers the height to 1 13⁄16 blocks high. This is tall enough to allow creepers to spawn, but short enough to prevent other mobs such as zombies and skeletons. In Bedrock Edition, this mechanic may not work. Be aware some Creeper farms can also spawn Spiders. This is easily avoided by removing 3*3 areas in the farm.