Zombie Spawner XP Farm

A zombie spawner farm is built exactly like the skeleton version: seal the room from light, use water to sweep mobs to a drop shaft, finish them off near the bottom. The differences worth knowing are all about zombie-specific quirks rather than the core mechanic.

Zombies occasionally spawn wearing armor or holding a weapon, somewhat more often than skeletons spawn with bows depending on version and difficulty, and any of that gear has a chance to drop on death. Worth checking the collection chest now and then instead of assuming it's rotten flesh all the way down.

Baby zombies are the real gotcha. They move a lot faster than adults and can slip through gaps a fall-based design assumes only adult mobs will use. A half-slab or trapdoor layer at the top of the drop shaft, specifically meant to block babies while letting normal zombies through, fixes the most common way these farms leak mobs into places they shouldn't be.

A soul sand and kelp bubble elevator sometimes gets added, not for the funnel itself but for your own convenience getting back to the surface quickly from what's often a fairly deep dungeon or mineshaft.

Rotten flesh piles up fast from this farm and it's worth more than people assume. Some villagers will trade for it at a poor but nonzero rate, so a big stockpile isn't purely something to toss.