This seems very do-able as a DYI project.
Cat castles & more
Interestingly, you can cut up an old carpet and hot glue it to the cardboard, and it adds strength. You can make a pretty fantastic cat castle fairly cheaply this way.
Hmm...this looks easier than the Imperial Walker
This is a nice cardboard cat house. Not too difficult to make.
Creative repurposing of drawers
eek, love this for my pup! Now, I have some ideas for my next DYI project. -jz
Cat tower boxes, not a bad plan.
German Cat Ladder