Walls of reality

The walls of reality, (TV: Rise of the Cybermen ^[+]Loading...["Rise of the Cybermen (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)"]) However, 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 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)"])