Cardboard
Given the price, I didn’t expect the inside to be made of cardboard. After a while, it starts to smell damp, even though I air out the house for hours every day. Since it’s cardboard, I can’t even wash it. I wash the cushions (but after one wash they lose their softness and comfort, and the filling is gone), but the smell stays. The tower is sturdy, although it wobbles when they jump on it.