Paris causes Achilles downfall just as he did in the Illiad, it's a dramatic juxtoposition of character. For some reason they left this crowd angering event in tact, although it is well telegraphed in the film. They changed quite a few other things, including killing Agamemnon, Achilles' implied homosexuality, and completely writing out the role of the gods. Also the trojan war was originally about a decade long, but those are just a few points among many. If you want a real story, not a stylized popcorn commercial that only appeals to teenagers, read the Illiad.