Opening a few miles from its namesake, The Great Wall introduces a group of European knaves who have somehow trekked to northwestern China during the Song Dynasty (960-1279). Most prominent among these thieves and mercenaries is William (Matt Damon), who's supposed to be British, although the actor doesn't further burden his stiff line readings with a feigned brogue. The outlanders' goal is to acquire some gunpowder, a Chinese invention with solid commercial prospects in war-happy Europe.
All the other rascals except Tovar (Chilean actor Pedro Pascal) are quickly dispatched. While being chased, the last two interlopers find their path blocked by that big wall and the troops that protect China from the menaces on the north side of the barrier. There are hundreds of color-coordinated warriors, but the important ones are strategist Wang (Hong Kong cinema lifer Andy Lau) and battalion commander Lin (Jing Tian, soon to be seen in Kong: Skull Island).
A lithe if blank beauty in skin-tight battle togs, Lin seems to have hatched not from Chinese lore but from Japanese anime. That's appropriate, since she and her cohorts defend China's capital from the Taotie, ravenous reptilian monsters with a strong family resemblance to Godzilla. (They also have zombie tendencies, and among the many credited writers -- all Westerners -- is World War Z author Max Brooks.)
This scenario may make The Great Wall sound entertainingly demented, but as so-nuts-it's-good spectacle, the movie disappoints. It's bonkers in theory, but prosaic in execution.