What made you dislike it?
First of all its a copy of, or rather inspired by Rashomon, a movie that i like, so i might be biased in that sense.
When it comes to the actual movie:
The separate story line was interesting conceptually but felt kind of "old" to me the third time with the Lady's perspective. Personally i found this repeatable structure to be more of a hindrance than an asset in this movie. The pacing also felt off at times.
The scenography was beautiful, the stage set in medieval France is very interesting to me as i love medieval history.
Therefore i was disappointed to see this movie become more of a "drama" based on 3 people, rather than a depiction or a portrait of what society was in that time and all its details. There is off course, some historical value in the movie, but it was much smaller than it could have been.
I wish they skipped the "drama" with the girl, the personal dramas etc and focused more on the society, structure and history. But i guess that is more of a personal preference anyway.
Acting:
Matt Damon to me did decent in this movie. He lacks charisma but that might be the point since he is supposed to be a brute anyway.
Adam Driver on the other hand did a very fine job and his character felt more complex and well put together. I wanted to know more about him as he seemed much more interesting.
The Girl had a pretty face but thats about it. Forgettable.
The duel at the end was intense and well put together, so a plus for that.
All in all a okayish movie , but it lacks punch and becomes to narrow for me to give it more than a 6/10 at best.
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I found it great, but I love history and this movie was probably one of the first to show the reality of european late medieval knighthood.
I do too and wish they did more of it rather than making it about a threesome drama.