Between the fortresses, the Germans constructed coastal artillery batteries, under the control of either the army or the navy. Spaced several kilometres apart, they were designed to fire out to sea and ward off any invasion fleet. They were equipped with guns (usually with a calibre of between 100 and 155 mm) which were generally grouped in fours or – more rarely – in sixes.
In all, there were more than twenty main batteries along the coasts of the Seine Bay between Le Havre and Cherbourg. Each of these was protected by a defensive perimeter ringed with minefields and a network of barbed wire, with machine-gun, mortar and anti-aircraft gun positions, connected by trenches.
Originally placed in open concrete pits, the guns proved vulnerable to Allied aerial bombardments, which had considerably increased in frequency since 1943. In order to protect them, Rommel ordered them to be placed in thick concrete casemates. This operation was far from complete by the spring of 1944, and as a precaution, some guns were discreetly removed from their emplacements and hidden inland.
On D-Day, the German coastal batteries offered only feeble resistance to the Allied ships, which overcame them without too much difficulty.