The Doctor mentions that the Brigadier mistook The Spanish Ambassador for The Master. This is an in-joke - Roger Delgado played The Spanish Ambassador Mendoza in the ITC Entertainment series Sir Francis Drake (1961).
Terrance Dicks has frequently stated that he disliked the original premise of the Doctor being trapped on Earth, and had meant to subvert this plan as soon as he felt he could get away with it. He recalls in a DVD documentary interview (on the "Inferno" release) having had it pointed out to him by Malcolm Hulke that the format limited the stories to merely two types: alien invasion and mad scientist.
This is the only serial of season eight in which Captain Yates and Sergeant Benton do not appear.
The location shoot in Cornwall was made difficult by cold, driving rain and resulting mud. Amongst other difficulties, the inclement weather caused the destruction of the garden planted outside the colonists' dome.
Unusual for the TARDIS, it is shown popping out sight. Director Michael E. Briant admitted that he didn't know it was supposed to fade out.