It might be fun. It would only follow the cultural borders of the city in which it is built, keeping barbarians away from that area. That effect would disappear with the discovery of Gunpowder, together with the ability to generate Great Generals at twice the normal pace. And there would be no generation of espionage points at all. Instead, Hadrians' Wall would generate culture for the city throughout the game.

Personally I think as well as the option to build city walls there should be a wall tile improvement that could be built between two tiles. Instead of a national wonder that affects only a city, you could build walls wherever you liked to keep barbs out. I think the great wall is currently a little overpowered, stopping barbs entering ANY of your territory, even after you have expanded from where the original wall was. Perhaps a cultural bonus for the biggest wall, and a great leader bonus for the largest enclosed walled area.

Quite good idea and does not seems really overpowered. As you can only have 2 per city, then you would build this only in a strong need.

Sometimes I feel invisible.