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The Seventh Door was a gateway that allowed living beings physical access to the Matrix. It was thought legendary. It could only be opened by the Key of Rassilon, which the Keeper of the Matrix held.

Towards the end of the Sixth Doctor's trial, the Valeyard fled into the Matrix through the Seventh Door using a copy of the Key of Rassilon once his identity was revealed by the Master. He was followed by the Sixth Doctor and Sabalom Glitz, the former of which exited the Matrix through the Seventh Door after he thwarted the Valeyard's plans. (TV: The Ultimate Foe)