I know this reply is a bit late, but I haven't been on for a while...
I went and researched the Battle of Thermopylae, and found a few discrepancies with the story the ads for the movie portray. For one, the Persians wanted to destroy Athens, not Sparta. There were also a few hundred local soldiers who stayed with the Spartans to help with the defense. There is also the small point that the Persians didn't have any mythical creatures in their army. And I would hope that the Spartans eventually all get slaughtered by a Persian force coming down the pass from behind them.