Built from 1939 to 1942, thirteen concrete observation towers were erected to defend the entrance to Delaware Bay during World War II, with eleven along Delaware beaches from Fenwick Island to Cape Henlopen and two near Cape May, New Jersey. They served as the eyes for the guns of Fort Miles, which was quickly built at Cape Henlopen to protect Wilmington and Philadelphia from Nazi naval threats, while an underwater minefield stretched from Cape Henlopen to Cape May. The towers were not armed; instead, observers used optical instruments such as azimuth sighting range finders to track enemy warships, telephoning coordinates to a battery commander station, where triangulation with known distances between towers allowed gunners to determine artillery fire. Soldiers climbed ladders through the centers of the towers, which had wooden floors, glass windows, winter heat, and room for four to six men on each floor. Built on pilings with reinforced concrete poured in a continuous stream and intended to last about twenty years, the towers remained standing after sixty years, though abandoned since the early 1960s, and the guns of Fort Miles were never called upon to fire at an enemy ship.