Let me throw out another option~ a catwalk! We put up carpeted shelves around our living room, to take advantage of the unused space, the kitties love it and it saves floor space in our small house! You are only limited by imagination, we have added hammocks, ramps and window shelves over the years.
Very cool! I like the way you put alternating shelves on the walls. I did this also but a bit differently...same concept. used wood, batting and fleece or sherpa...here are some pics...added two poles for climbing...lots of vertical space. The pics don't contain the second poll yet and I will be getting rid of the cat tree/condo and replacing with a new one.
part ll of this post is now here: http://www.savannahcatchat.com/threads/so-you-want-to-build-a-cat-walk.819/