So I spent yesterday in the nether building a wither skeleton farm. 53 blocks long by 31 blocks wide and 5 blocks high. Floors of nether brick I mined from a fortress. Ceiling of cobblestone and walls of mostly blue glass. Dug straight out of the netherrack. I was seriously injured by lava multiple times and once accidentally hit a pigman with my pickaxe and had an uprising on my hands. But I finally finished and it was magnificent! But nothing was spawning in it except for zombie pigmen. Nothing. So I Googled and learned that I was 100% wrong about the spawning rituals of wither skeletons - I thought they would spawn on nether brick wherever it was. But actually they only spawn inside nether fortresses regardless of whether the floor is nether brick or not. So there the structure sits - an empty, worthless white elephant of monumental proportion. It will sit there abandoned by all but the zombie pigmen until the end of time.