Built from 1939 to 1942, thirteen concrete observation towers were erected to help 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 at Cape Henlopen, part of a wider coastal defense system that also included an underwater minefield stretching to Cape May, while Fort Miles itself was built in two years to protect the industrial and shipbuilding cities of Wilmington and Philadelphia because forts farther up the river were considered too distant for effective defense. The towers were not armed; instead, observers used optical instruments, especially azimuth sighting range finders mounted inside the towers, to track enemy warships, take readings every 30 seconds, and send coordinates by telephone to a base station bunker or gun battery, where triangulation was used to direct artillery fire. Built on pilings driven into the sand and formed from reinforced concrete poured continuously, the towers ranged from about 39 to 75 feet tall and were designed to last about 20 years, yet many remained standing after 60 years. Once equipped with ladders, wooden floors, glass windows, and electric heat, they were later abandoned, and only the concrete remained. The guns of Fort Miles were never called upon to fire at an enemy ship.