With the new gameplay videos we got quite a new look on the Great Wall improvement. Unfortunately I just found filthyrobot's videos with actual china gameplay, but he didnt build the wall. So we dont know exactly how big the defense bonus is and how big the yields of it are.
But we got still new informations. First, the GW in the prerelease build just needs the research masonary, which is a very early tech compared to the tech for military engineer who can construct forts later in the game.
Alone that gives china a massive def bonus compared to other civs, only if you look at the problems you can have with the barbarians early on. And it might be a good idea to rush archery so you can defend your walls with archers, who are now vulnerable and the wall def bonus might help them.
And in the tooltip of the current version of the pre release lets plays it is now mentioned, that you get an adjacency bonus for other wall tiles connected and a bonus when the wall is built on hills.
So it might be very helpfull in the early game to build the wall as long as possible, so you have to put your cities closer together, so the borders will connect faster. Especially if your gold bonus will get higher then 1 it can be very strong compared to other gold income ressource. Yeah, gold might be easier to obtain early now at least by city gold income, but you can use it more now to pump out more units which are quite needed it looks like.
And yeah, it can be used aggressively, just place one tile on a hill for an archer to begin your offense and than you can quickly forty your new lands.
Im still on the search for some deeper gameplay of china especially with someone who actually builds the wall, but I found none so far ...
