Was the Great Wall of China effective in defending China?
While the Wall probably stopped casual incursions by small groups of raiding nomads, the best that could be hoped was that it would slow down large armies and give the Empire a chance to mobilise its defences. In fact, the troops and officers assigned to the Wall were notoriously ineffective, and there are a number of accounts of them betraying gates to the invading nomads. In addition, the cost of keeping the Wall manned and repaired was high, and the Empire often did not want to afford the expense. In short, the costs probably outweighed the benefits.
