Romaji: Kuppa Ōkoku) (also known as Dark Land or Castle of Koopa) is monarchy ruled by Bowser since childhood serving the last worlds in Mario.
As the name implies, is a birthplace of Koopa population which the landscapes consist volcanos, lava rivers, wastelands, dark clouds, forests, dead trees, poisonous mushrooms, undergrounds lavas and rocks. They were found bones in plains and caves including Yoshis, Goonies, Humans and Koopas in some games. The danger this place is not only the lava, but also the lava geysers, volcanic debris and volcano smogs (may be the cause of these skeletons in dry plains and caves showing that did not survive because of smog ashes). Here is located the Bowser's Castle which is the capital this kingdom and home of Bowser and his family. Strangely exists icy parts this kingdom located in underground caves which melted because of magma for years showing by vapors, but some were preserved of heat.
In contrast to other kingdoms of Mushroom World which are peaceful and friendly, Koopa Kingdom always involves in wars whose ruler wants to expand his kingdom invade others. Mario and his friends sometimes invades that kingdom for rescue Princess Peach most of times, recover something stole by Koopa Troop or stop Bowser's plans.