In the TARDIS, the Doctor's regeneration is failing. He retreats to the tranquil environment of the Zero Room, little realising that he is snared in a trap set by the Master. The evil Time Lord has kidnapped Adric, and used a facsimile of the boy to send the TARDIS hurtling back in time, to be torn apart in the Big Bang. The desperate solution to the situation results in the destruction of the Zero Room, so Tegan directs the TARDIS to the mountaintop city of Castrovalva, legendary for its serenity. There she hopes the Doctor will be able to recuperate from his recent trauma... but has the Master laid a trap with a trap?

During the summer of 1980, Doctor Who producer John Nathan-Turner and script editor Christopher H Bidmead began to suspect that Tom Baker's seven-year tenure as the Doctor was coming to an end. To help audiences make the transition to Baker's successor, they conceived a trilogy which would bridge Seasons Eighteen and Nineteen. The arc would be linked by the involvement of a rejuvenated version of the Doctor's Time Lord arch-foe, the Master. He would be reintroduced in Johnny Byrne's The Keeper Of Traken, cause the Doctor's regeneration in Bidmead's Logopolis, and then face the new Doctor in the nineteenth-season premiere. Originally, this was to have been “Project Zeta-Sigma” by John Flanagan and Andrew McCulloch, the team responsible for Season Eighteen's Meglos. However, in February 1981 -- with production less than two months away, and Bidmead having been succeeded by trainee Antony Root -- it was determined that major elements of “Project Zeta-Sigma” were highly impractical, and replacement scripts would have to be found.

This would normally be cause for the utmost panic but, early in 1981, the Doctor Who production team learned that they would have the luxury of several extra months before the transmission of Season Nineteen. Peter Davison, who had been cast as the Fifth Doctor, was already committed to a second season of the sitcom Sink Or Swim, to be recorded while the production of Doctor Who was paused in June and July. Sink Or Swim was intended for broadcast in the autumn, and the BBC was wary of overexposing Davison by featuring him as the lead in two series airing simultaneously. As such, it was decided that Doctor Who would be held back until January 1982 -- the first time since 1974-75 that the show would premiere in the winter rather than the autumn.

The BBC was studying the public appetite for twice-weekly series, and deemed Doctor Who a good experimental subject

Furthermore, Doctor Who had fared very badly in the ratings throughout Season Eighteen, as competition from ITV's glossy American import Buck Rogers In The 25th Century devastated its audience. Alan Hart, the Controller of Programmes for BBC One, concluded that Doctor Who's traditional Saturday teatime slot -- which it had occupied since its debut in 1963 -- was no longer appropriate for the series. At the same time, the BBC was studying the public appetite for twice-weekly series, as they prepared to launch the soap opera which would ultimately become EastEnders. Given that Doctor Who was a well-established programme, the BBC deemed it a good subject for experimentation, and so it was decided that Season Nineteen would air across Monday and Tuesday evenings. On June 27th, The Sun was the first to report on the programme's move from Saturdays.

As a result, Nathan-Turner enjoyed unprecedented flexibility to rearrange his recording schedule. He had now come to the conclusion that Davison would be better served if he could tape several stories before tackling his debut adventure, hooking audiences with a more fully-formed Fifth Doctor at the start of the season. It was therefore decided that the serial chosen to replace “Project Zeta-Sigma” would be made fourth, after Four To Doomsday, The Visitation and Kinda.

To write the new premiere, Nathan-Turner and Root contacted Bidmead, who had returned to freelance writing after leaving Doctor Who. They requested a story featuring the Master in which the new Doctor would be suffering from regeneration trauma, induced by the entropy field encountered in Logopolis. It was felt that this turmoil would help establish the Fifth Doctor as being more vulnerable than his seemingly indomitable predecessor. For his part, Bidmead wanted to resume the exploration of the TARDIS that he had begun in Logopolis, while also contemplating the role played by perspective in distinguishing between reality and illusion.

A starting point for Bidmead was his recollection of Nathan-Turner's irritation at the optical illusions depicted in two prints which hung in the office of Graeme MacDonald, the Head of Series and Serials. The images had been drawn by the Dutch artist and mathematician Maurits Cornelis (MC) Escher, whose works included Relativiteit (or Relativity, 1953), in which figures walked up or down all sides of a series of staircases; Belvedere (1958), in which the perspective of a building changed between floors; and Klimmen en Dalen (or Ascending And Descending, 1960), in which a staircase endlessly looped back onto itself. Bidmead thought that an environment exhibiting these sorts of traits could complement a story which dealt with the mathematical concept of recursion, in which each member of a sequence is generated by one or more of the preceding members.

The MC Escher lithograph Castrovalva depicted a village atop a sheer cliff

On March 9th, Bidmead was commissioned to write a storyline called “The Visitor”. While developing his ideas, he was inspired by a 1930 Escher lithograph called Castrovalva, a landscape which depicted a village in the Abruzzo region of Italy, sitting high atop a sheer cliff. Bidmead decided to use a similar image in his narrative and, by the time he was asked to provide full scripts on April 8th, the title had actually become Castrovalva. By this point, Root had left Doctor Who and Nathan-Turner himself was serving as the programme's script editor until the beginning of Eric Saward's appointment in the middle of the month. Nathan-Turner instructed Bidmead to motivate the changes to Nyssa's costume which had been introduced in Four To Doomsday, including the replacement of her skirt with corduroy trousers, and the elimination of her tiara and fur stole. He also indicated that Bidmead should depict the Doctor's addition of a celery stick to the lapel of his cricketing jacket.

The director assigned to Castrovalva was Fiona Cumming, who had worked as both an assistant floor manager and a production assistant on Doctor Who on several occasions since 1966. She and her team travelled to East Sussex for location filming, which began on September 1st. Although Crowsley Park in Berkshire had served as the grounds of the Pharos Project in Logopolis, the Crowborough Wireless Telegraph Station in Duddleswell was now used for this purpose, creating some continuity errors -- regarding the placement of the TARDIS, for example. An unintended consequence of making three other serials prior to Castrovalva was that Cumming had to encourage the series regulars to behave with less familiarity towards one another.

September 2nd and 3rd were spent at Buckhurst Park in Withyham, where many of the Castrovalvan exterior scenes were filmed. Upon arriving at Buckhurst Park, Nathan-Turner greeted a man he assumed to be the gardener -- only to discover that he was actually addressing the estate's owner, William Herbrand Sackville, the tenth Earl De La Warr. Despite this case of mistaken identity, Earl De La Warr was delighted to have Doctor Who filming on his premises; in lieu of receiving a fee for the use of his property, he had agreed to a charity donation and a photo in front of the TARDIS. Earl De La Warr also threw a party for the cast and crew on the first night, during which alcohol was liberally available. Matthew Waterhouse indulged rather too greatly, and found himself feeling very poorly during the next day's recording. Indeed, immediately after shooting Castrovalva's final scene, he proceeded to vomit against the nearest tree.

Cast and crew visited three Groombridge locations during the last day of filming, on September 4th. First, the stony rise to Castrovalva was actually Harrison's Rocks. Cumming had to shoot this material carefully due to Janet Fielding's severe fear of heights, framing the action to make it appear as if the actors were at a much greater elevation than was really the case. Next, the area where the TARDIS materialised was situated in nearby Birchden Wood. Finally, the sequences involving the Castrovalvan hunting party were filmed at Aytton's Wood.

Studio recording then began on September 15th and 16th at BBC Television Centre Studio 3 in White City, London. This block was dedicated to scenes inside the TARDIS; the console room set was in use on both days, while the second day also included work in the Zero Room and the TARDIS corridors.

The Castrovalva sequences were Cumming's focus during the second studio block, which took place in TC6 between September 29th and October 1st. Recording on the first day involved scenes in the time travellers' rooms on Castrovalva, alongside modelwork. The production was visited by Patrick Troughton, who had played the Second Doctor; he was invited to stand in for Davison during camera rehearsals. On the second day, material in the village square and on the walkways was recorded. Cast as the young girl was Souska John, the niece of Caroline John, who had played companion Liz Shaw in 1970. The final studio day was spent on the sequences in the Portreeve's chamber, the Castrovalvan entrance tunnel, and the Master's TARDIS.

John Nathan-Turner insisted that Peter Davison be credited as “The Doctor” rather than “Doctor Who”

Unusually, Anthony Ainley was effectively credited twice on Castrovalva Episode Three. To avoid spoiling the fact that the Portreeve was really the Master in disguise, Ainley was listed as playing the evil Time Lord, while “Neil Toynay” was billed as the Portreeve. In fact, this was an anagram of “Tony Ainley” devised by Cumming's husband. Meanwhile, Nathan-Turner insisted that Davison should henceforth be credited as playing “The Doctor” rather than “Doctor Who”, breaking with a tradition that dated all the way back to the programme's very first episode.

The opening installment of Castrovalva was broadcast on January 4th, 1982. It marked the return of Doctor Who to British television after an absence of more than eight months -- the longest in the programme's history to that point. On Mondays, Doctor Who aired at 6.55pm between the news and The Rockford Files (although So You Think You Know What's Good For You? filled in for the James Garner mystery series on the evening of January 11th). On Tuesdays, Doctor Who was preceded by the news and a cartoon short, and followed by the quiz show A Question Of Sport. Its regular timeslot was 7.05pm, although the omission of the animated programme on January 5th meant that Episode Two aired five minutes earlier. In both cases, Doctor Who was replacing the twice-weekly nursing drama Angels. Not every BBC region followed this schedule, however. BBC Cymru decided to run the news programme Heddiw in the Doctor Who timeslot. As a result, Welsh viewers watched Doctor Who at 7.45pm on Mondays and Wednesdays. To the north, BBC Scotland screened Episode One at 3.30pm to accommodate the documentary The Heavies.

• Doctor Who Magazine #258, 19th November 1997, “Archive: Castrovalva” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #1, 2001, “Prince Charming” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #34, 2016, “Story 116: Castrovalva”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Handbook: The Fifth Doctor by David J Howe and Stephen James Walker (1995), Virgin Publishing.
• In·Vision #55, January 1995, “Production” edited by Anthony Brown, Cybermark Services.