Some people don't know that wither skeletons can spawn anywhere in the fortress bounding box as long as it's on netherbrick! It makes for an amazing mid game skull farm. : r/Minecraft
Minecraft Wither Skeleton Farm - Over 60 Skulls Per Hour - 1.16 - YouTube
EASIEST Auto Wither Skeleton Farm Tutorial! 130+ Skulls/hr! | Best Early Game Minecraft 1.17 Farms - YouTube