Its a series of many walls (which didn't stop the mongols) and the fortifications are a result of work done over 2,500 years.

It isn't against the laws of physics to build a wall, but that isn't the point either. The point is mass deportations have occurred frequently in history and in a very short time span vs earthworks so large and grand that the roman empire rose, peaked, and dissolved inside the entire time frame in which the "wall" was being built. Any walls built are where they're most needed and everywhere else is empty field that might, might not be monitored by drones and patrols.