The film is based in a series of novels written by former Spanish war correspondent Arturo Pérez-Reverte. He had the idea for the books when he had a look at his daughter Carlota's History book from school and saw that only one page was devoted to the 'Siglo de Oro', the years in the 16th-17th centuries when Spain was the world's dominating superpower. Carlota, then 12, helped her father research the period, and the first novel, published in 1996, was published with 'Arturo y Carlota Pérez-Reverte' as the author. Five novels were published before the film was shot, and the film is based in the most important episodes in all of them... and beyond. The sixth novel was published in Spain in December 2006, and Pérez-Reverte has said he has drawn some inspiration from the film for the upcoming novels.
During the opening of the Battle of Rocroi, the matchlocks muskets are firing without the serpentine or "hammer" holding the match moving. To fire a matchlock the burning end of the cord/match must swing down to the priming pan by the side of the matchlock.
When the group walks back from their white flag meeting with the French commander at the battle of Rocroi, the movie focuses on how few Spanish soldiers are left. In the foreground a few casualties can be seen. On the left a man in brown clothes lying on his back, his spear in his right hand. A little to the right in this same shot exactly the same casualty in exactly the same pose can be seen, except for a few details regarding the shadows on the ground.