I've asked the same question in an other thread about this movie.

my remarks were:

-all that fire (fire arrows that keep burning) and use of gun powder while it was clearly raining heavily.

-the movement speed was enormous and that unit of knights that just went straight through that unit of infantry (not the least resistance, like it were ghosts)

-the highly accurate 'big bertha'. One shot fired 1 unit of knights dead.

I'm fairly sure that the cannon was just for dramatic effect and that in the game the accuracy will be far less.
The movement speed and the lack of weather effects (apparently no noticeable effect on gameplay) makes me more concerned. Let's just hope it was just a movie to show off and not a real (staged) battle.