Constructed to appear to be a defended bunker, it is not far from the Atlantic Wall NN8303 : Atlantic Wall
It can be seen on these aerial photos - Link
Real-life enemy Tobruk design here Link (Archive Link )
I understand these walls and bunkers were built for training purposes in WW2. Troops were training for the D Day landings. It is reported that there is a concrete bombing target arrow in the area pointing towards the wall, which suggests that it was also used as a target for aircraft taking part in exercises.