Therefore, the challenge is to create a scenario where a pre-gunpowder European civilization builds a structure similar to the Great Wall of China, although almost certainly not anywhere near as long as that wall. The wall has to be at least 100 miles long--in other words longer than Hadrian's Wall. I imagine the Romans are the ones most likely to pull this off but am unsure where they'd build it--Germania? The challenge is creating the right combination of economic prosperity and a need for long fortifications to get the wall built.