This hill houses two strongpoints. 140, Stp Osnabrück was occupied by the Luftwaffe and gave shelter to a radio guidance system. In the battle for Britain the German bombers were guided home after their missions over England. The X and Y-Verfahren had strong radio transmitters, placed in several locations along the coast. The receiver in the airplane used these homing signals to assist the pilot plotting the route back. The second strongpoint is 139, Wn Hamm, which was an artillery commandpost.
It looks old, but we are not sure that it is German.