 Zenithia, also called Zenith Castle, Zenith, or Tenkū-jō (天空城, "Heaven Castle") in Japanese, is a floating castle that first appears in Dragon Quest IV; it is used as a descriptor for several elements in Dragon Quest IV, V and VI. Its appearance in all three games has led to the games being described as the Zenithia or Tenkū trilogy, despite different geographical layouts in each of the three games' worlds. Horii explained that a trilogy was never the intention: "Each Dragon Quest title represents a fresh start and a new story, so it seems too much of a connection between the games in the series. It could be said that the imagination of players has brought the titles together in a certain fashion."
