I found a website for a cat rescue that actually built their play area around this huge tree. It had a glass ceiling so the tree could still get sunlight and they must have had 40 cats in and around that tree! Does anyone know the name of that rescue?
Anyway, I think that is a little more that what you are talking about
But, your idea sounds great! We're going to screen in part of our deck for the kitty's but they will only be able to use it in summer. I wish I had the room for something like that all year round.
Are you going to board cats, or is it just for you babies?