Another excellent book to screen adaptation was the Green Mile... accurate almost to the last detail (with the exclusion of a few minor characters for pacing).
As for Troy, they could have farely easily made a realistic movie with a 10 year siege through a single time lapse shot. It‘s a simple enough device and is used in films all the time. As for Achilles‘ love interest, they probably thought that the character needed to be softened down a bit; show that he would show mercy to his enemies, and yet at the same time, not show mercy to his enemies (Hector). If you went to the movie looking for an accurate depiction of the legend, or something that was even remotely historically accurate, then you will no doubt be as disappointed as I was.
As far as just being a movie, it wasn‘t bad. Lots of action, minimal plot or character development, but enough momentum to keep it interesting. However, I had a really hard time believing that a city of thousands would be willing to wage a war over a couple who are about as vacuous as Ken and Barbie... seemed more like an adolescent crush rather than true love.
Oh, and Helen of Troy was NOT a blonde!