Artistic License - Biology: The Black Death is portrayed as a leprosy-like disease that causes boils all over the face and hideously disfigures its victims. In reality, the disease, by most accounts, only caused swellings in the lymph glands (the throat, armpits and groin).
The Atoner: Behman feels guilty over accidentally killing a woman during a siege. The Demon later mocks him for thinking that saving the accused girl would atone for the far greater number of innocents he's killed.
Burn the Witch!: Averted in the prologue where three accused witches are shown being hanged, then drowned in case they're Not Quite Dead. Unfortunately for the priest carrying out the ritual, one of them definitely isn't.
Butt Monkey: Through the course of the movie, the all ready unfortunately named Debelzaq gets stabbed through his palm with his own cross, rope burns on both of his hands, a lot of accusations of molesting the witch. And he dies without much of a fanfare
Cardboard Prison: It turns out the girl could have escaped from her cage at any time.
Evil Plan: The villain is a demon that seeks the only book with the knowledge to banish it back to hell in order to destroy it and the entire movie is the heroes helping them find it. Also a case of Using You All Along.
No Name Given: The witch, though it's finally subverted at the end of the film when she's exorcised and introduces herself as Anna.
Not Even Bothering with the Accent: Nic Cage and Ron Perlman play 14th century Europeans with broad American accents. Bizarrely inverted with the supporting characters, most of whom were played by British or European actors. The director apparently instructed them all to speak with American accents, presumably to blend in with the lead actors. Unfortunately, none of them were using the same American accent. And very few of them were using it consistently.
In TV spots, it is revealed that the witch turns into a demon
The beginning of the movie implies that the girl might just be a misunderstood innocent caught in a witch hunt. Anyone who's seen a trailer for the movie knows this isn't the case.
The trailer gives away the fact that the inhabitants of the monastery have been turned into zombies.
Use Your Head: Felson is quite fond of this. Unfortunately it doesn't work on Demons.
Xanatos Gambit: The heroes are charged with taking their prison to a monastary for trial and possible execution but whether they arrive safely or not works to their prisoner's advantage. If any of them try to kill her before they get there, out of paranoia or whatever, murder is a cardinal sin. If any of them die before reaching the monastary, the Demon has less interference to destroying the book needed to send it back to Hell. The heroes are leading them right to it.