This was originally planned as an American-set film to star Steve McQueen and was announced as such by Twentieth Century Fox in 1963.
Despite having several British actors and crew members, this film never got a cinema release in Britain. It had a brief video release in the early 1980s (luridly re-named "Lunatic") but is mainly known in the UK following TV showings and a DVD release (under its proper title) in the 21st century.
The first director attached to this project, in the early 1960s, was Robert Siodmak.
This film was made in Scandinavia, but it is never actually stated that it is set there (or anywhere else). Several characters are either not named or else have names which could belong to any one of several countries.
Salem likes to play chess. Max von Sydow, who portrays Salem, plays chess with Death in The Seventh Seal (1957).