Hello, The Nazis were aware that an invasion would happen. But, by the use of false intelligence during Operation Fortitude, the Allies were able to trick the Nazis into believing that their invasion of mainland Europe would happen in the North of France or Norway. German fortification along the coast was known as the Atlantic Wall. It was a series of bunkers that guns pointed into the sea, tank traps on the beaches and almost 6 million mines laid around Northern France. They were supported with division of infantry and armor with the ability to be ready when the invasion started. As they believed that the invasion would occur in the north, the Nazis did not have a lot of division by Normandy. After D-Day, false information was sent to the Nazi's high command that D-Day was just a diversion in order to send Nazi's re-enforcements from the north to the south. There were several fortresses built along the Atlantic Wall with a garrison of anywhere from 3,500 soldiers to 90,000 soldiers.