I have applied The Great Wall to Chinese Civilization. However, there is a small problem; because of the game mechanic for this wall while it is applied to Campaign only. It is not flexible like the normal walls we build. You need to make a SQUARE concept like a fortress or a straight line.
The Great Wall is really nice and big. Thus, I increased the HP, cost and build times for these objects.
After that I wanted to use normal Chinese Walls via Chinese Wood walls / bastion / gate which I repeated what I had done for Russian Civilization to make Russian Civilization to have stone walls. However, I realized something while doing lots of testing. When you change your wall concept to The Great Wall, the stone wall bastions and wood bastions are using the same bastion parent data model, so I couldn’t add that flexible Chinese Stone Wall Bastion to wood based bastion model exchange.
At my mod thread in the AOE4, I had explained the problems I experienced while applying this Great Wall, it seems like developers made some small limitations. Once you get used to this Wall construction method, it is quite fun to build the Great Wall. Your defense siege experience become totally different in the gameplay really^^.
I wanted to add Kiev Walls to Russians; instead of west style; but some models are hidden that content editor doesn’t let you build, it is same for Wolf Monument etc too 