William (Matt Damon) and Tovar (Pedro Pascal) are adventurers/mercenaries/bandits in China during the Song Dynasty, in search of "black powder" (gunpowder, not available in the West at the time). They speak modern American English - a little odd in 1050 AD. They end up on the Great Wall, which turns out to be defending China against the Taotie (evil, intelligent lizard-like creatures much larger than a man). The very large army along the wall is "The Nameless Order," consisting of colour-coded troops of warriors. Zhang Yimou's obsession with colour really lets him down here: the colours are anachronistic (they couldn't possibly have been created in the time period), far too consistent, clearly computer-generated, and make the troops look as if they're wearing plastic.
If you can get past that, and the odd need for the Chinese to find a white guy to help save them (or did they just want his help at the North American box office?), it's just an epic war movie with a not terribly interesting plot. The characters are both overblown and weak - partly because the writing is bad and mostly because the concentration is on the spectacle of it all. Don't waste your time.