This "...reinforced concrete mock-up of an anti-invasion Atlantic wall..." was used during World War 2 for training.
See here Link
Some information about THE Atlantic Wall here - 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.