There's two ways to view The Great Wall, and I don't know which is the correct one. Either it's a completely serious piece of essentially chinese state film propaganda about a legendary monster, or its a dumb self knowingly silly action film.
If it is indeed the overly serious one, then The Great Wall is to use the technical term, a load of old rubbish, bilge, whatever. If its a dumb fun film, its kinda ok fun. Seen a lot worse. Along those lines.
Matt Damon doing what he thinks is a European accent is a mercenary in early China with a Spanish guy who does matador tricks to fight monsters, who when looking for gunpowder get captured by a secret chinese army department who man the great wall of china (how can that be secret then?) to defeat monsters who come along to punish China for the greed of its emperor a thousand years ago. Together with Jian Ting (Might be spelling that wrong) as a female army commander and Willem Dafoe as a fellow European in the wall, they have to find a way to defeat the monsters.
So its as simple as it can be, yet also overly convuluted. Shocking.
Anyway, back to the possible meaning of the film. Whilst the chinese cast (which is the majority of the film in fairness since this is a chinese film) are portrayed as honourable, clean living, trusting, hard fighting warriors, the europeans of the cast are displayed as ugly, dirty, smelly, sneak thieves, sly and dishonourable. Honestly, its super weird. There's several points where characters have discussions about their different value sets, and the way the Chinese army is portrayed as being so much superior to the Europeans not only in weapons and tactics but also in morale and beliefs, its pretty amusing really. I mean if this what the Chinese government considers effective propaganda, its laughable nonsense. If.
But if we take it as a dumb action film and ignore the huge leaps of logic, since its a film about monsters and weapons, its pretty ok. The monsters aren't very interesting in their behaviour or design, but they're decent enough cannon fodder to be murdered. There's weird logic leaps when extremely useful weapons aren't used until certain points in the film, and there's a severe lack of hand to hand combat which is dissapointing. The acting is also pretty terrible. Well I say that, its kinda hard to tell with a lot of the chinese cast, while Matt Damon and the european characters are boring and rubbish.
On the other hand, it looks pretty nice overall. Whilst the European characters are all pretty murky and dirty, the Chinese army has these spectacular looking uniforms in all different colours to denote which part of the army they're in, and it looks pretty neat in the battle scenes. The best parts of the battle scenes are these really, when it goes pretty mad and you can embrace it, coloured costumes doing battle with monsters.
So, the plot is either terrible nonsense... or terrible nonsense. The action is decent enough overall, even while being silly. And the acting is stilted and bad. Overall... not a recommendation!