Astacia Kingdom is one of the two main worlds in which Falling Through takes place.

It is one of the various nations that reside in Dio's world, and is the main kingdom in the story. It is ruled by the Royal Family, and the majority of the population consists of nymphs and amalias. There are few humans and even fewer elves in the kingdom, as they are often discriminated against by others. They live mostly in the outer region of the kingdom in the slums. Most of the aristocrats live in the area that surrounds the Royal Castle and are mostly made up of nymphs, including the Kyanite Family.