Here is a report that was taken by the Military Intelligence Service on the 15th June 1943, which stayed classified until 23rd April 2003.
The Germans have installed gun emplacements in caves and hollows in the cliffs along the French coast. It is assumed that these positions have been considerably improved with concrete. Such positions, for machine guns as well as for artillery, may be found, to cite one example, in the 2-mile stretch of steep cliffs between Lion-sur-Mer and Asnelle-sur-Mer, northwest of the city of Caen, France. Cave positions proved highly effective in the German defense of Dieppe during the Allied raid. After a smoke screen drifted away and uncovered the attackers, machine guns and artillery opened up on them from positions concealed in caves in the cliff face. Some evidence was found to indicate that 88-mm or French 75-mm guns were in these positions. The guns were impossible to detect even at close range until they fired.