The Forbidden City is the resting place of both the Oracle Crystal and the Oracle. It is an ancient underground city, notorious for how difficult it is to enter, and the numerous booby traps that lay inside. As Piper quotes from I.J. Domiwick's book,"The builders of the city believed in tricks and hoaxes. First impressions are always wrong, and nothing is what it seems."
In the episode "Forbidden City", I.J. Domiwick, an explorer, is seen helping Snipe to find a way into the Forbidden City to retrieve the Oracle Crystal. Predictably, Snipe betrays Domiwick and tries to claim it for himself. Aerrow sets the Oracle free and runs with the Oracle Crystal just before the city collapses.
In the episode "The Key", Cyclonis and the Dark Ace head to the Forbidden City to retrieve the Oracle Crystal which is revealed to be the key to the doorway to the Far Side of Atmos. Cyclonis summons the Oracle to find out where the key is, which is in the hands of Piper. When Cyclonis threatens to destroy the Oracle, Aerrow and Piper step in to give the Oracle time to escape. In the battle, Piper accidentally drops the key and Cyclonis manages to escape with it. In the end she is seen opening the doorway and walking in to the otherside.