Quite possibly the only Giallo thriller filmed in Ireland.
The films opening credits state it is based on the novel "A Room Without a Door", by Richard Mann. Italian film historian Roberto Curti said the novel was an invention of the filmmakers. The screenplay for the film was written by Sandro Continenza and Riccardo Freda, while other credited writers André Tranché and Günter Ebert were credited solely for co-production reasons.
Riccardo Freda was unhappy with the film and had his name replaced with the pseudonym "Willy Pareto".
When the German co-producer Artur Brauner saw the finished film, he decided to skip a theatrical release in Germany, due to the quality of the film which he wasn't satisfied with.
Prior to the release of Dario Argentos film The Bird with the Crystal Plumage (1970), giallo films were not popular among Italian film audiences. Following the release of The Bird with the Crystal Plumage, a wave of giallos were released with animals' names in their title. This led Riccardo Freda to follow suit and attempt a film in the genre with The Iguana with the Tongue of Fire (1971).