I made a world download for you to explore. My question is this: can wither skeletons spawn in the very back of the chambers? (shown in the picture) They have a light level of 5. Most people said that there is a bounding box, and this farm is right above a 4 way bridge, so I dont know if this fits within the box. Anyway, heres the world download: https://www.dropbox.com/s/5p645mapotxx1sr/The%20Prototype.zip?dl=0
If you've built your spawning chamber where a corridor or walkway was then it doesn't matter what blocks you have on the floor (as long as they have a solid top surface) but in the rest of the area of the fortress they will only spawn on nether brick.
According to the Wiki wither skeletons, like most hostile mobs, spawn if the light level is 7 or less.
But they probably spawn more often in lower light levels.
Odd. Why does the link I've inserted send me back to this topic?
I don't know, I just get a blank page.
And right clicking on it doesn't let me check where it links to.
Cutting and pasting into the URL box in Chrome does take me to Dropbox and the world download.
What do you mean by "above" a bridge?
I'm not sure how much higher the bounding box goes, I'd use the actual surface of the bridge to be on the safe side, just knock down the railings and widen it a bit. If you widen it with nether brick you should be able to make it practically as wide as you want (as long as you stay within the fortress area.)
I tried pasting in your link in this post just to test and I got a forum error message.
It might just be the forum that's mixing it up somehow.