The poster says the Great Wall took 1,700 years to build. This is technically accurate, since it was actually constructed as a series of several walls built separately over that time by various ruling dynasties. Each new wall section built by any single ruler would take approximately ten to 20 years to construct.
Andy Lau and Matt Damon both played the same role of an undercover gang member working in the police force. Lau played a triad member and senior inspector working for the Hong Kong Police in Infernal Affairs (2002), and Damon played his American counterpart, Staff Sgt. Colin Sullivan of the Massachusetts State Police, in its American remake The Departed (2006).
Not knowing the entire Yimou Zhang filmography. The following might be considered directors trademark or intentionally linking both movies The Great Wall and Ying Xiong (Hero). They both start with introductory text ending with the sentence: This is one of the legends. Also, the main character of Hero is called Nameless the same as the army in The Great Wall that is called Nameless Order. Among other common points such as the use of colors to separate characters/stories.
The trivia item below may give away important plot points.
When William (Matt Damon) was asked if he had been left behind before, he replied, "Yes. Twice before. And it didn't end well for them." This is an homage to his previous films Interstellar (2014) and The Martian (2015), in both of which he was left behind on other planets and the expedition went wrong.