Tokyo is a city on Earth, and the place where most of the Sailor Moon series takes place. Serena, Amy, Raye, Lita, Mina, and Darien all live in Tokyo. All the forces of evil seem to attack here for some reason, whether it is the Negaverse gathering energy, Alan and Ann gathering energy, the Bureau of Bad Behavior seeking out Pure Hearts, or the Dark Moon Circus searching for dream mirrors.
The Negamoon Family, on the other hand, targets Tokyo for a specific reason; In the future, Tokyo will be put under an evil spell, but will be saved by Darien and Serena when they rise to the throne, and turned into Crystal Tokyo, which the Negamoon Family were unable to destroy, so Rubeus was sent into the past with the Negamoon Sisters to take over crystal points in Tokyo. All his attempts failed, and after his death, Emerald took over, with plans for using Dark Wedges to fill Tokyo's vulnerable spots with dark energy. All her plans failed as well. The Dark Crystal was eventually moved to Tokyo, but was destroyed when Serena and Rini defeated Wiseman.