From the Super Mario Wiki
The Koopacropolis is a platform situated at the top of a large hill, and it is used as the hideout of King Koopa in The Super Mario Bros. Super Show! episode "The Fire of Hercufleas". Not much of the Koopacropolis is actually seen, though what little is seen indicates that the Koopacropolis is little more than a fortress on a mountain, the inside of which is fairly plain in appearance. After he is defeated, King Koopa and his minions abandon the Koopacropolis.