As it turns out, the Chinese used a different wall-building technology than contemporary Europeans, creating walls that were highly resistant to gunpowder artillery. While European walls relied on bricks and mortar, Chinese walls were of tamped earth construction, i.e, they used a labor intensive process that relied on compacted earth to build truly massive walls. These were notable for being capable of resisting not only 18th century naval bombardments, but even 20th century Japanese artillery.
A basic suggestion might be, first, to rename the Great Wall Chinese Castle Age technology to Hangtu Walls, Hangtu being the Chinese term for “rammed earth”.
A more advanced and questionable suggestion might be to change it so that Chinese walls resemble those seen in the “Into China” campaign; i.e, Chinese walls now cost 2 stone and build 233-400% faster (taking 2 or 3 seconds to build), but only have 50-55% the HP. This would help make them more tactically flexible as well, since they now build significantly faster than Palisade Walls and are thus suitable for gold-plated combat use, with 900-1500 HP depending on whether Fortified Walls are researched.
The drawback, of course, is that if Chinese walls are thicker, Chinese BBT + Fortified Wall placement would have to change, but I’d think the change would be interesting as it’d force players to use different walling strategies for the Chinese.
There’s also the downgrade problem, but we could also consider giving the Chinese a second UU, this time a structure instead, of a Hangtu Wall, that requires the Hangtu Wall research.