Douglas Adams, who wrote the script under a pseudonym, reworked the story into his novel "Dirk Gently's Holistic Detection Agency", which also used characters and situations from the uncompleted Dr Who serial "Shada", which he also wrote.
Tom Baker found filming in Paris to be a very different experience to what he was used to in the UK where crowds would gather to watch the filming and meet the stars. Doctor Who (1963) was not shown in France at the time and so the cast and crew were largely ignored.
The original intention was to recreate Paris in a studio. It was John Nathan-Turner who pointed out that it would be much easier and cheaper to actually film on location.
The artist who sketches Romana in the café was originally a much more prominent figure called Bourget, who is in league with Scarlioni.
The title is possibly a play-on-words of Cité de l'amour (City of Love, as Paris is often known), and Cité de la mort (City of Death), the pronunciation of which is very similar.