- The driving force behind the film was 'Burton, Richard' 's son, who wanted to see his father in a good old-fashioned adventure movie. Burton approached producer Elliott Kastner for ideas, who asked Alistair MacLean. At that time, most of MacLean's novels had either been made into films, or were in the process of being filmed. Kastner persuaded MacLean to write a new story. Six weeks later, MacLean delivered the script.
- Clint Eastwood was reluctant to receive second billing to Burton, but agreed after being paid $800,000.
- The "Schloss Adler" is actually the "Schloss Hohenwerfen" in Austria. At the time of filming, the castle was being used as a police training camp.
- An accident during one of the action scenes left Kastner and director Brian G. Hutton badly burnt.
- Despite Eastwood's reputation for violence in other films, his character kills more people in this film than any other Eastwood character.
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