Michael Crichton, author of the same-titled book on which this movie was based, disliked this movie so much that he refused to licence any more movies based on his novels. Nobody would gain the movie rights to a Michael Crichton book until Steven Spielberg, long-time friend of Crichton, bought the rights to "Pirate Latitudes" after Crichton's death.
Michael Crichton took the unusual step of offering up the movie rights for free, provided the movie entered into production immediately. Crichton had a back-end deal set up on the basis that his script be used for this movie. Ironically, Crichton ended up hating the finished movie.
The battle of Castlegard is not an actual historical event. However, the crew of the movie visited various European castles from around that period (the late 1300s) to make the castles and towns look as realistic as possible.
A group of medieval re-enactors were used for soldiers in battle sequences.
The problem of language is ignored in this movie, having a French-speaking character. In the novel, however, it is stated that, as in reality, people in 1357 did not speak modern English or French, but Occitane, a language in which Andre Marek (Gerard Butler) is fluent, according to the book. Also, the novel describes some high-tech translator devices the characters put in their ears, which also is absent in this movie.