- Completely Different Title:
- The French title is "Descente au Paradis" (Descent into Heaven).
- The German title, "Die Angst des Doktors", translates as "The Fear of the Doctor", continuing the theme of The Something of The Doctor from Series 7.
- The Hungarian title is "Az örökkévalóság másodperce" (Second of Eternity).
- The Japanese title is "Captured by Shadows". Previously, the Japanese titles of previous episodes of Series 9 inverts this trope by giving direct translations of the episode titles.
- Edited for Syndication: A variation: BBC America apparently realized too late that this episode was extra-long, but rather than dropping scenes, they time compressed that is to say, very slightly sped up note it so it could run in a 60-minutes-with-commercials time slot. The following week's repeat and later "Doctor's Notes" version (with text commentary from fans, behind-the-scenes tidbits, etc. added) were each given a 75-minute time slot so it could air in its original form.
- Recycled Script:
- The premise is similar to that of a script Steven Moffat mentioned he once considered pitching as a Big Finish Doctor Who story with the Eighth Doctor: The Doctor being stuck alone in an Eldritch Location with a threat that preys on his fears. This episode would fit right in within the Divergent Universe Story Arc, with the cyclical nature of the story and its dark tone. Torture, body horror, and the Doctor trapped in a place for far longer than the audience was expecting has happened to Eight often; "The Last" (a Divergent Universe story) has an ending in which the Doctor realizes that he has to die by killing himself, he reboots the universe! It also has similarities to Robert Shearman's episodes "Scherzo" (the first Divergent story) and...
- Steven Moffat's favourite Sixth Doctor story in all of Doctor Who, "The Holy Terror". Which also features an immense castle with no real "outside" stuck in a "Groundhog Day" Loop that functions as a Psychological Torment Zone and always ends with the prisoner's death. Only this time, the Doctor's the prisoner instead of having wandered in.
- The Doctor being alone and repeatedly attacked psychologically and physically in a mental landscape comes to us courtesy of "The Deadly Assassin", the only classic series serial that had no companion character. Perhaps coincidentally, "Deadly Assassin" also followed the departure of one of the show's longest-serving companions who, much like Clara, had become beloved by many fans and by the Doctor. And it also involved the Time Lords messing around with the Doctor's life.
- The story also has possibly-intentional similarities to "Logopolis" (a mysterious figure seems to herald the Doctor's death) and its immediate followup "Castrovalva" (the Doctor is trapped in a castle made of Bizarrchitecture constructed by someone with connections to the Time Lords).
- This isn't the first time we've seen the repeated re-spawning of short-lived individuals who struggle and die in the name of a nigh-impossible goal, only to generate equally-doomed successors to continue that endless struggle. Both sides of the conflict in "The Doctor's Daughter" were doing the same thing throughout their war, albeit for seven days rather than 4 1/2 billion years.
- In one of the comics, the Doctor meets Prometheus (the actual Greek god), who's been imprisoned but says he can escape by pounding on the wall for thousands of years.
- What Could Have Been: During the 2020 COVID 19 lockdown rewatch, Steven Moffat had said on Twitter one of the original concepts of the story was the Doctor trapped in a Haunted House with the Weeping Angels.
- Word of God: In a Radio Times interview Steven Moffat clarified several things, such as the fact that Clara's portrait was painted by an earlier Doctor and that the Wall never resets because it's part of the Confession Dial outer wall, and not the castle itself. He also revealed that in his mind the first few iterations of the loop went very differently and lasted much longer, until some things such as the clue to look for Room 12 slipped through the dodgy resetting process and allowed for a tighter and more consistent loop that we see in the episode. Further comments in Doctor Who Magazine revealed that one of the "I Am in 12" messages is on the back of Clara's portrait, but the Doctor stopped looking for it there as he couldn't bear turning Clara's face to the wall.
- You Look Familiar:
Trivia / Doctor Who S35 E11 "Heaven Sent"