On all of the strategy guides I have gotten, they say the great wall acts as city walls in every city....I don't see this in the descritption on the game, and I don't see city walls in my city after I build it...
The Great Wall acts as a city wall in every city even if you don't see city walls on the screen. You can also still build regular city walls when you have the Great Wall, so you can prepare for metallurgy which obsoletes the Great Wall.
look at your city icons.You will see a little wall surrounding each city.They can't be sabotaged either.Not that the AI would try to do that with any organization.