Ive also wondered about this. Does it only count with the cities that are surrounded by the walls our all the cities that you posses. Secondly I notice that when I build the wall the borders expand very slowly so I was wondering how does this work. How do you get the walls to expand to the greatest distants. I notice when i reload the walls will spread out further but is there any other trick. How does this work?
The walls you see are just a pretty graphic. The effects of the GW extend throughout your culture borders, even if your culture borders are not continuous (e.g., you have a couple of cities off by themselves).