The Mamodo World, referred to as Demon World (魔界 Makai) in Japan, is the homeland of all mamodos (魔物 mamono, lit. "demons"). The Mamodo World can be seen as a world like Planet Earth, but with it's own habitats, life, and magical awareness. The Mamodo World was predominantly featured in the movie Konjiki no Gash Bell!! Movie: Unlisted Demon #101 but was briefly seen in the flashbacks of the anime and manga.
The Mamodo World is ruled by a single ruler, a King or Queen, though the ruler can also have a King or Queen by their side. The Mamodo King governs over the entire Mamodo World for a thousand years. A new King is announced every thousand years among one-hundred chosen mamodo children candidates.
The elementary school that Zatch Bell and his goes to is on top of a huge plateau that can be seen over miles. The Headmaster of the school administers the thousand-year battles. Mamodos learn various things from regular school subjects to magic classes, teaching children how to use spells and other forms of magic, like psychokinesis to move or manipulate objects with the mind.
While the Mamodo World is a magical place, it's natural life is somewhat similar to the Human World. Some life that appears in the Human World also appears in the Mamodo World, such as yellowtails and tamagoyaki that grows on trees.