The upper villager ends up getting killed eventually now. Not sure why.

By the way, with where the cats are positioned, if a creeper spawns in the very corner block on one of the pads, it will just get stuck in that spot forever.

I was hoping this might be expandable if you just made more floors, but the mobs end up landing on the trap doors on the way down and dying, leaving their drops where they landed.Ã

By the way, I know everyone does this here, but the efficiency of the grinder isn't really based on anything realistic. On any multiplayer world where you'd need grinder drops, you likely won't ever be able to build it where no other mobs will spawn in the area, and you probably don't want it to be blatantly visible for people to grief.

Great grinder got about a stack of gunpowder in about 2 minutes and can also be an exp grinder if you add a couple of blocks to where the mobs fall