In the thirties the forts of the outer ring were modified. The old 5,7 cm turrets were replaced with concrete positions. However the glacis plates from the turrets were maintained to give additional protection. That is why most of the concrete positions have disappeared, because the Germans scrapped all the remaining metal. However Fort Lier was fortunate. When the Germans blew up the first turret of the fort they discovered that it had no glacis plates. So they concluded that the other turrets had no glacis plates too (which in fact they did) and left them undamaged.