SAO: Aincrad is an iron-and-stone made floating castle that has about ten kilometers floor base diameter and consists of a hundred floors stacking straight upwards, meaning that each floor's diameter is a bit smaller than the previous one. On each floor, there are a couple of large cities along with countless small scale towns and villages, forests, plains, and even lakes. Only one stairway links each floor to another, and the stairways exists beyond the boss room of each dungeon. The Teleport Gates of every unlocked cities are connected to one another. ALO: The World Tree, also known as Yggdrasil, is the massive tree growing at the center of ALfhiem. It was said that the first race to reach the legendary city, floating above Yggdrasil, and have an audience with the Fairy King Oberon, who lived there, would be reborn into true fairies, the ALF. The hover limit imposed by the system would be eliminated and unlimited flight would become possible for the players. Along with it, the successful race would be capable of becoming rulers of the infinite sky. It was said to be possible to enter this floating city through a dome at the root of Yggdrasil.