“I was immediately drawn by the idea of fighting monsters on the Great Wall,” says Zhang. “I found that a very unique idea.”

For Zhang, a landmark originally designed to maintain peace and prosperity in China was the perfect setting “to protect the world”.

You might wonder why the story stars a white male in Matt Damon. His character, William Garin, is a mercenary fresh from the Crusades who has travelled the Silk Road with a group of bandits harbouring the intention of stealing one of China’s most valuable inventions: gunpowder. But after Garin gets captured, he becomes unwittingly embroiled in this fight to save mankind against the creatures.

When the first trailer dropped over the summer, Zhang immediately faced a firestorm over Damon’s casting. Social media users, perhaps prematurely, claimed that Damon’s role should’ve gone to an Asian actor.

Others complained that the film was propagating the “racist myth that [only a] white man can save the world”, as actor Constance Wu, star of the comedy show Fresh Off The Boat, wrote on Twitter. “Our heroes don’t look like Matt Damon,” she added.

Zhang says he feels “this is a great misunderstanding between the audience and what we put out.”

“They [the critics] were guessing what the movie was about and they guessed wrong. But it’s really unfair – this negative information has got spread out, so far and wide, all across the globe. Everybody in the world has been asking me this question. I feel it’s really unfair to Matt Damon – it hurts him because this is not what the movie’s about.”

It doubtless explains why Zhang, 65, recently created an epic nine-minute trailer – as if to carefully clarify exactly Damon’s role in the movie. “The idea that he stole an Asian or Chinese actor’s role is preposterous,” says Zhang.

“We were never intending to cast a Chinese person in that role. And then the idea that it’s only white heroes… that’s preposterous too. There are many Chinese roles that are very heroic. They didn’t wait for Matt Damon to show up before fighting this monster!”

That doesn’t explain why Damon is front and centre in the film’s poster and other promotional material. But Zhang had other problems of a logistical kind. Unable to shoot on the real Great Wall, his team built two walls – one low, one high – in a studio in Qingdao. Both stretching to well over 200 metres, and a giant green screen was placed behind each to aid the computer-generated effects.

“Matt Damon said it was the biggest green screen he’d ever seen,” says Zhang, “and I think it’s the biggest green screen ever assembled in the world.”

Shooting began almost two years ago, followed by over 12 months of labour-intensive post-production with Weta, the New Zealand outfit behind Lord of the Rings, responsible for providing all the digital effects, as well as producing over 20,000 props and 5,000 weapons for use on the set.

With a crew compromising of over 1,300 personnel from 37 countries, one of the most staggering statistics is that the production carried over 100 translators, communicating in multiple languages.

“It is much harder to direct in a language I don’t speak,” says Zhang, who – despite working with Damon and co-star Willem Dafoe here and Christian Bale on his last film, The Flowers of War – speaks minimal English.

“The best way is to find actors who are naturally good actors and also actors that you can trust,” the director says. “So after having a very detailed discussion with them about the characters, you can trust them to deliver something honestly.”

With a cast that includes Hong Kong’s Andy Lau Tak-wah, one of the more intriguing elements for Chinese audiences is the casting of Wang Junkai, the lead singer with popular Chinese boy band TFBoys, as the young emperor.

“We needed to find an actor for the emperor role,” Zhang explains. “I thought, ‘Why do we need a grown-up man? Why don’t we have a young emperor?’ Historically, they were [young]. So I reached out to Wang Junkai and he was ecstatic.”

The singer also known as Karry had never acted before, but Zhang denies it was stunt casting.

“Of course I tested him rigorously. We did a lot of screen tests and I found him to be very smart and very natural. We also gave him some acting training and once he got on set, he was so relaxed. He did his role so vividly; he was good at taking direction.”

Whatever it means for Chinese cinema as a whole, The Great Wall is undeniably a production that is taking the Hollywood approach. Merchandise tie-ins – everything from porcelain tea sets to cellphone cases and even a fashion range – are already saturating the shops. It’s the sort of activity that would impress any studio head from America.

Still, it seems all too fitting that the Taotie, the mythological monster scaling this ancient landmark in Zhang’s film, is also the symbol of greed.

The Great Wall opens in China on December 16, and in Hong Kong on December 29

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