When Matt Damon was cast in veteran filmmaker Zhang Yimou's new adventure The Great Wall, the internet erupted with cries that putting a Western actor into the story was "whitewashing" Chinese history. But the actor was quick to point out that the movie is a fantasy and his character was written to be Western.

"The whole idea of whitewashing, I take that very seriously," Damon says. "This was designed as a giant East-meets-West coproduction creature-feature monster movie!"

Matt Damon in The Great Wall

The plot centres around European mercenaries who travel to China to steal gun powder, then end up helping the locals battle a horde of monsters. "Those allegations were kind of centred on the poster," he adds. "I think people maybe thought it was something about building the Great Wall, or something having to do with actual Chinese history. But it's this crazy monster movie set in 1100 AD."

Damon says his character also helps clarify the issue. "William is basically a barbarian who is exposed to this civilisation that is way more advanced than anything he's ever seen," he notes. "He is really moved by what he sees and the values expressed by this civilisation."

And then there's the film's wacky mythology. "Yeah," Damon laughs, "the idea in this movie is that the Great Wall was actually built to keep this wave of monsters out and they attack every sixty years. The only thing protecting northern China from these monsters is the wall and the army that guards it, so there is this fictitious army we've created called the Nameless Order. They are born into it and spend their whole life training for the fight. If these monsters break through, they are basically going to take over the world eventually." He goes on to explain that the monsters, which are called Taotie, come from Chinese folklore and represent greed.

More: Watch a series of featurettes on The Great Wall

And for Damon, the best thing about making the movie was filming on the Great Wall itself. "There's no way to describe the Great Wall," he says, "and friends who had been there told me that. It's a bucket list item. Everyone wants to get there but, walking up with my wife and kids, it's almost unimaginable that human beings made that. It just goes on forever. It goes and goes and goes and was made by hand so many years ago. It's spectacular. I can't compare it to anything because there is nothing like it."

Watch the trailer for The Great Wall: