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With the first kitty, I purchased a three level, green (from the olden days) carpet covered, cat bed/tree. It lasted for decades and many cats used it along the way. With the next kitty, I purchased a more natural colored, dual level cat tree that had a bed on the top. This also lasted for decades. Meanwhile, the green bed/tree went to my sister's cats.
With kitty number three, I got another duel level natural cover tree/bed. This had a round bed with a hole for the kitty to come up to the top through the lower level. While it looks interesting, almost like a piece of sculpture, the cats didn't like it near as much. They used it, but more to run across to get away from the other cat than as a bed or something to climb. It was sort of a tweener, inbetween the favorite tree and the living room chair.
Today, my cats have a multi level cat tree with a bed on top. It is about 6 feet tall. They love it for napping, scratching, and playing on. Occasionally one of the cats will pat someone on the head when they walk by. I am kinda thinking that the taller the tree, the more the cats like it, and that's what it is for; to keep our little furry friends happy.
Regardless of what cat tree you get, it is handy to have one of the bottom sections covered in sasal rope. The rope is wound tightly around one of the legs or panels. It encourages them to sharpen their claws on the cat tree instead of your victorian velvet chair. The good thing about the placement of the sisal rope on a lower section is then the cats can not only sharpen their claws, but also strengthen their back muscles and keep their leg muscles strong.