In the scene where the witch Mary Collins declares revenge on Uther, the actress, Eve Myles, gave such a performance that the French extras (who didn't understand a word she said), applauded spontaneously, and Anthony Head (Uther) demanded to reshoot his lines, saying "I'm not good enough!"
In the original draft of the script; Arthur and Merlin's first initial fight intended the characters to be shirtless, this idea was later changed, because the production team thought it was too severe and strong for a playful first fight.
During the scene where Merlin looks out a window after arriving at the castle, the music is note-for-note from "X-Men: The Last Stand."
Most of the main cast of this episode have appeared in Doctor Who (2005). Colin Morgan (Merlin), Angel Coulby (Gwen),
Anthony Head (Uther Pendragon), Richard Wilson (Gaius), John Hurt (voice of Dragon) and Eve Myles (Mary Collins/Lady Helen).
Colin Morgan (Merlin) and Bradley James (Arthur) have both worked with actors Tom Goodman-Hill and Rebecca Front. Colin worked with them both in Humans (2015), while Bradley worked with them in an episode of Inspector Lewis (2006) that he appeared in.