Atreia is the "planet" in the Aion universe where the game is played. Unlike planets of our universe, Atreia resembles two hollow hemispheres (or rather, "hemi-shells") with a single axle tower, the Tower of Eternity, connecting the centres of the two halves. Life flourishes inside the shell-like bowels, while the "surface" of the world fosters unique and dangerous forms of life. Prior to the Millennium War, all life giving energy was supplied by Aion, shining forth from the Tower of Eternity in the centre of the planet.