The bottom part is not a solid piece, the walls are fabric that you velcro on which I did not expect. We have 4 cats so I'm alittle concerned about the durability of it however they are enjoying climbing up it. Two of our cats are around 14 pounds so it's nice that this has larger platforms for them. It does wobble a bit when they jump on it, but that seems to be the case with any cat tree we have brought home so we just add  weight to the bottom.