The driving force behind the film was Richard Burton's stepson, who wanted to see his stepfather 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.
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. There are no cable cars near Schloss Hohenwerfen. Hence the Cable Car shooting is done somewhere else.
In the scenes where Richard Burton and Clint Eastwood climb the steep fortress walls, Burton moves with ease, while Eastwood is clearly working hard physically. This was due to the fact that Burton, who was a hard-drinker and out-of-shape by that point, chose to ride a crane (made invisible by special effects) up the wall, whereas the young, healthy Eastwood was actually climbing the wall.
The part that ultimately went to Clint Eastwood was also offered to Lee Marvin, but he declined, telling the producers they were about 4 years too late. Marvin had already starred in a WW2 action-adventure, The Dirty Dozen (1967), which he hated. Although it made him a huge star, he did not want to return to that type of movie.
In a recent Channel 4 (UK) survey of the top 100 war movies Steven Spielberg voted this as his favorite, mainly due to its sheer "boys own" factor of unreality. He even went so far as to repeat the "Broadsword calling Danny Boy" line.
The name Mary is given when introduced as Heidi's cousin, is Maria Schenk. Maria Schenk was the middle name of Colonel Claus Philipp Maria Schenk von Stauffenberg who was the chief conspirator in the July 20th plot to assassinate Adolf Hitler. It was Stauffenberg who placed the briefcase with the bomb under the table at the Wolf's Lair.
Even though Alistair MacLean wrote both the book and the film, several characters have different names in the book and the film respectively. Curiously some sources (e.g. Screen World) refer to these names in the cast list.
The trivia item below may give away important plot points.
Derren Nesbitt was nearly blinded when the squibs in his chest blew upwards instead of outwards when filming his death scene - his character was filmed being shot in the head and the chest but in the finished film he is only shot in the head.