Why can’t I fill an unlimited number of construction spots?
The number of spots available and thus the size of your castle depends on your level. As you progress and level up, additional construction spots are unlocked. This way, your castle grows along with your experience.
Exception: Decorations directly attached to the castle do not use up any construction spots! However, decorations that are placed next to the castle (such as the windmill or the horse) do use up construction spots.
After opening the construction menu by tapping on the "Build" button, you can see the number of available and used construction spots.