In preparation for the expected invasion of German-occupied France, extensive fortifications were constructed by the Germans along the northern and western coasts of France. Work on the defenses, dubbed the "Atlantic Wall," began in March of 1942 and consisted of a variety of walls, bunkers, minefields, barbed wire fences, trenches, observation posts, artillery positions, beach obstacles, and other defensive constructions. Not knowing where the Allied forces might attack, Germany was forced to cover as much of the 1,670 miles of French coastline as was feasible.
The Germans suspected that an invasion would take place at the Pas-de-Calais region of France because the English Channel was narrow at this point, and the area was closer to the heart of German war industry in the Ruhr-Rhine region. As such, Pas-de-Calais was more heavily defended than other parts of the Atlantic Wall.
The fortifications were constructed by Organization Todt, which used a combination of military, civilian, prisoners of war, and slave laborers for its projects. Rommel had little authority over Organization Todt, which was run by Albert Speer, Minister for Weapons and Munitions. In spite of their efforts, the Germans were never able to construct as many defenses as they had planned. This was due in part to a shortages of vital construction materials such as concrete, as well as insufficient labor to construct the defenses in the time allotted.
Field Marshal Erwin Rommel was appointed commander of Army Group B in 1943, and set about to reinforce the Atlantic Wall in accordance with German doctrine that called for the swift and absolute defeat of any Allied landings.
Although Rommel oversaw additional defensive measures, such as the placement of hedgehogs, wooden ramps and "Rommel's Asparagus", and to the flooding of areas in which airborne landings might take place, the Atlantic Wall had no true depth to its defenses. An enemy force that breached the thin Atlantic Wall would face no further fortified positions of significance.
German propaganda had succeeded in convincing many that the Atlantic Wall was impenetrable, but in fact it was highly vulnerable and proved as such during the Operation Overlord assault on June 6, 1944.
One key vulnerability of the defenses was the lack of appropriate ammunition for the various captured artillery pieces employed along the coast, most of which were of French, Soviet or Czech origin. The quality of troops manning the Atlantic Wall was inconsistent, and most suffered from shortages in fuel and transportation. These problems, along with Allied naval and air superiority, doomed the Atlantic Wall long before a single Allied solider hit the beaches.