The Royal Navy waived royalty fees on the use of stock footage and clips showing ships in action, happy with on-screen credits and the positive publicity generated by the show. Many sailors volunteered to help with the filming, so that most of the extras during the sequence at the Naval Base were actual service personnel, except in some of the stunts.
The children's TV show with pink knitted aliens being watched by the Master (that he jokingly pretends he thinks is real), is The Clangers (1969).
The sea fort setting was a late change to the scripts. Malcolm Hulke had originally written this material for an oil rig, but Michael E. Briant was unable to obtain permission to film on one.
The script called for Jo Grant and the Doctor to climb up a ladder to get into a sea fortress. The ladder proved too slippery for Katy Manning, so stuntman Stuart Fell did the shot dressed as Grant.
Barry Letts regarded this as one of the most successful serials he produced and credited much of this to the dynamic direction of Michael E. Briant.