Hot take: Eternia is a poorly designed toy for the space it takes up in your house.
It's about 48 inches wide and about 36 to the top of Central Tower's cannon. For all that, you get 3 decent sized rooms, a jail cell, the periscope room and a large and small parapet for figures to play in.
Convert that footprint to a basic dollhouse design and you could have 12 inch wide (width of a room in Castle Grayskull) columns of rooms 4 levels of 8-9 inches high each plus playable roof space. There could still be good, evil and neutral ones separated by 6" wide shafts that could have trap doors on top and could function as an elevator shaft or have a first floor room for a monster to live.