Static defenses evolved from the use of permanent fortifications that were direct descendants of medieval castles. As artillery improved in destructive power and penetrative ability, more modern fortifications were developed, using first thicker layers of stone, then concrete and steel. After naval artillery developed the turret – a moving cannon platform – land fortifications started to use this method as well. Between the World Wars, France built an "impregnable" underground steel and concrete fortification that ran the length of the German-French border. This Maginot Line failed to stop German tanks in 1940: they bypassed the fortifications by invading through neighboring Belgium.As artillery and rifles allowed the killing of enemy personnel at a longer effective range, soldiers started to dig into temporary fortifications. These included massive trenches as used in WWI, and individual soldier-sized "fox holes" which became more common in WWII.