- The protagonist fights a big monster. This illustrates that good triumphs even against overwhelming force. The Dark Tower had this in the scene where Roland kills the demon.
- The protagonist fights a lot of evil minions. This illustrates that good triumphs over the amassed efforts of villainy. The Dark Tower had this in the fight against the Man in Black's minions in the village.
- The protagonist fights the film's avatar of evil, and wins. This illustrates that good inevitably carries the day against evil. It isn't much of a spoiler to tell you that Roland kills the Man in Black, right? Anyway, I'm doing you a favor so you don't have to watch this movie.
Bonus fourth convention:
- If the film features a naive, young protagonist there will be a scene in which they narrowly escape the clutches of corrupt agents of the greater evil. This illustrates plucky resolve in the face of depravity, or some such thing. The Dark Tower had this in the scene where Jake runs away from the fake boarding school employees.
Think about that formula, the narrative structure it serves, and where you find it in any number of action-fantasy films--both good and bad. Of course, the difference between The Dark Tower and any number of better movies that use the same structuring conventions is that the better films have the decency to obscure their generic conventions with interesting characters, plot development, and non-wooden dialog.