Looks like we’re all pretty much in agreement now with the caveat that Mathochist went ahead and used 6 inch walls and the rest of our answers use the no width vertical walls from the OP. I hope you’ll tell us more about the book when it’s released.
Wow! Lots of answers! Lots of **different ** answers…
Which, actually, I find comforting. If it takes twenty posts worth of calculations and discussions by the smart people who hang out here to settle on the answer, then it was entirely reasonable for mathophobe me to throw up my hands without really trying.
To address **aahala’s ** concerns, no, of course none of this math will appear in the book. In fact, none of the answers are likely to appear as definite numbers. What I wanted to be sure was that I had a reasonably accurate mental picture of this setting so that I wouldn’t have things happening that were impossible or ridiculous.
For example, suppose the ‘central’ section of the upper floor turned out to be only 2 feet wide. I’d feel plenty stupid if I used part of it as a child’s dormitory and talked about rows of beds sticking out from the wall, now wouldn’t I?
Just to double check, here is my mental image of this area, numbers rounded off as ‘good enough’ for ordinary purposes of writing:
Main floor: Both floor and ceiling are approximately 19’ wide. The walls belly out a bit, with roughly an extra foot of width at chest height.
Bottom floor: has a sloping floor, making walking around on it more and more difficult the further you get from the center line. Various electrical conduits, water mains, and so forth are spread out and affixed to the ceiling over head. Working on the mechanical systems is a PITA (and likely involves much head-bonking) for anyone over 6’ and also for those who are shorter, depending on the size of the particular system and how far ‘out’ from the center it runs. (I may decide to not have the side walls on this level at all.)
Top floor: The central chamber is roughly 11 feet wide, with a ceiling height that rises from 4 1/2 to 6 feet in the center. The entire width is easily used by children, but adult caretakers will need to do various amounts of crouching as they get nearer the side walls.
The side chambers are roughly four feet wide, with a wall on one side that is 4.5’ and a ceiling that slopes down from that to meet the floor on the other side. Hmmm. Let’s see if my feeble math skills are worth anything: if we assume that the cross section of the side chamber were triangular, that would make the hypotenuse uh, 6’ close enough. Then the largest circle that would fit into it would have a diameter of something like 2.6. Right? And since the ceiling is actually a curve, there’d be a smidge more room…
So, can I safely say that any creature that could walk/fly/squirm/slither through a 2.5’ hoop can sneak around through the side passages? Yes/no?
Oh, excellent. Since at least some adult humans are less than 2’ wide, I can force some poor schmuck to crawl/drag his way through the passages in pursuit… 
I loooooove the way that works out…assuming I didn’t screw up on that last diameter.
My thanks to one and all!
Sounds good at a first guess, but I haven’t broken out the pencil and paper.
Mostly I wanted to jump on the fact that I mentioned a “beastie” a little while back…