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.
Quite possibly the only Giallo thriller filmed in Ireland.
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.
Riccardo Freda was unhappy with the film and had his name replaced with the pseudonym "Willy Pareto".
Director Riccardo Freda was not proud of the film and was unhappy with the cast. He later said that he had initially wanted Roger Moore in the lead, but at the last minute got Luigi Pistilli.