It was not a true military wall in the sense of what some think, HUGE walls, 30 feet high with Crenelations, etc. It was....a wall. It did an adequate job of keeping those out, but should the 'barbarians' assault on mass, it would have provided little adequate defense. Just like while most believe the Great Wall of China was built to keep the barbarians out, such was not the case at all,as the defensive features, the Crenelations and what not, face INWARDS towards China. As a result the Wall was more a means of keeping invaders IN where they could be pushed back against the wall, rather than keeping them out.