Planeworld is a pyramidal frustum (a pyramid with the tip cut off). The inner faces are habitable and home to numerous species. The exterior faces are inhospitable to all but the hardiest of creatures. Each face is 40km thick and contains extensive cave systems that connect the interior and exterior faces. The crust of each face is made of an incredibly strong material that is near impossible to penetrate, so the natural breaks in it are the only way to move deeper into or out of the face. There is a magical barrier between each face that prevents creatures from getting close enough to walk from one face to another.