I had 2 cardboard boxes in my living room that our 3 cats loved to lay in.  I decided the cardboard was a little trashy and we needed an upgrade.  One cat took to this immediately, almost to the point of being a bit territorial.  The top level and second level (open on both sides) are the favorite.  The bottom level seems to be the least favorite.  The openings are a bit smaller in real life than they appear in the photo, however my cats are all between 12-13.5 pounds and everybody still fits