Stackable. The top is mesh, so it comes with a heater, the Entotsu (REP Japan M size, one unit), which can be used on the ceiling and emits far-infrared rays. The heater was only used for one season. (The heater cost about 6000 yen) The cage was used for about a year. The heater was used for about 4 months (purchased 10 months ago). Used for a hedgehog. Selling because I'm moving to a new cage. There are a few marks from tape used to secure pet sheets inside the cage, but they are not noticeable.