When my cats were kits in 1998, I built a tower/playhouse out of 8" PVC water pipe (green) with carpet scraps on the inside so they could climb as well as the outside. They had favorite types of carpet. They loved the turns (tees and wyes), the suspended from it, etc. Working with an underground utilities contractor helped me get good prices on the pieces that were not already scraps from a job. I must admit the tower they have in this house is PetCo-purchased, but they love it. The plan that was posted in an earlier post looks great.
Yes, when I bought the way-too-expensive cat tower, my sweet little Aussie got a fancy feeder and waterer so she had "equal" treatment. Sadly, she went to doggy heaven in 2001. :-(
Glenna