Walls of reality
The walls of reality, (TV: Rise of the Cybermen [+]Loading...["Rise of the Cybermen (TV story)"]) also known as the walls of the universe, (TV: The Stolen Earth [+]Loading...["The Stolen Earth (TV story)"]) the walls of the world, (TV: Journey's End [+]Loading...["Journey's End (TV story)"], The Next Doctor (TV story [+]Loading...["The Next Doctor (TV story"]) often just the walls, (TV: Doomsday [+]Loading...["Doomsday (TV story)"], Wild Blue Yonder [+]Loading...["Wild Blue Yonder (TV story)"]) were barriers which surrounded the Doctor's universe and made travel to and from other realities challenging.
According to the Tenth Doctor, before the Time War, when the Time Lords "kept their eye on things", travelling between universes was easy, but after their passing, "the walls of reality closed, the worlds were sealed". Travel between parallel worlds became all but impossible, (TV: Rise of the Cybermen [+]Loading...["Rise of the Cybermen (TV story)"]) unless something caused the walls of the universe to break down. (TV: The Stolen Earth [+]Loading...["The Stolen Earth (TV story)"]) This was the case when the Sphere, the Dalek Cult of Skaro's Void Ship, managed to "break the walls" to return to their original universe after sitting out the end of the War in the Void. After the Doctor and his allies forced the Daleks back into the Void, the Breach they had created, and which Cybermen from Pete's World had exploited to cross over, sealed itself up again. (TV: Doomsday [+]Loading...["Doomsday (TV story)"])
The universal detonation of the reality bomb, which ultimately would have destroyed all of reality save the New Dalek Empire within the Medusa Cascade, caused the destruction of stars which was observed in Pete's World as it was ahead of the Doctor's universe. Rose Tyler used a dimension cannon to travel from Pete's World back to the Doctor's universe as the dimensions started to collapse not just in those two universe, but the whole of reality, with Rose finding that even the Void was "dead". In the Planetary Relocation Incident, the Children of Time prevented the detonation of the reality bomb, resulting in the walls of the world closing again, which the DoctorDonna identified as dimensional retroclosure. (TV: Journey's End [+]Loading...["Journey's End (TV story)"]) The Doctor was aware that the Cybermen and the Daleks that had been sent to the Void earlier had perished, but confronted a group of Cybermen that had fallen through the dimensions and back in time to 1851 whilst the walls of the world were weakened. (TV: The Next Doctor [+]Loading...["The Next Doctor (TV story)"])
The Fourteenth Doctor later noted that the edge of the universe was "where the walls are thin". Consequently, when he invoked a superstition at the edge, (TV: Wild Blue Yonder [+]Loading...["Wild Blue Yonder (TV story)"]) he symbolically "let in" the Toymaker, who had been confined to his own realm beneath the Under-Universe. The Toymaker created an "entrance", which remained even after he was defeated, with the Toymaker warning that "his legions" were coming. This warning (TV: The Giggle [+]Loading...["The Giggle (TV story)"]) was soon accomplished with the emergence of Maestro, the child of the Toymaker, and a fellow member of the Pantheon. (TV: The Devil's Chord [+]Loading...["The Devil's Chord (TV story)"])
