This is the second time Christopher Smith has made a film in Germany; parts of Creep were filmed there too. Despite being in English with an English cast and director, Black Death (2010) is essentially a German film.
In the original script by Dario Poloni, the second half of the film turned to be entirely supernatural: Langiva was revealed to be the reincarnation of the Devil, while Osmund "was actually in Hell and Hell being in the physical place.". Christopher Smith rewrote the second half, changing these elements to represent the idea that to him "Hell is the Hell you are in within yourself".
The trivia item below may give away important plot points.
The scene where Osmund encounters the body of Averill in the woods and then gets jumped by a pack of bandits was shot in just one hour. They were forced to shoot it so quickly as the light was fading quickly.