|The Space-Time Door|
The Space-Time Door (時空の扉) was the gateway to the Fourth Dimension in the Sailor Moon universe, and was guarded by Sailor Pluto and, for a brief time in the manga, Diana. The gate was accessible during both Silver Millenniums through the deeper levels of both the Moon Castle of the past Moon Kingdom, and the Crystal Palace within the future Crystal Tokyo. The physical location of the door did not seem to exist in the normal space-time in which the majority of the story took place.
In the anime, the door was decorated with the phases of the moon. It first appeared in episode 82, when Sailor Pluto opened the Door to allow Sailor Moon, Tuxedo Mask, the Inner Senshi, and Chibiusa to pass through.
The door was also accessible from any time and place if one carried a Key of Space-Time provided by (or stolen from) Sailor Pluto. This Key would allow a traveler to locate the Space-Time Door with ease through the otherwise troublesome and very dangerous Fourth Dimension.