Built from 1939 to 1942, the concrete observation towers along Delaware's coast formed part of the World War II defense of Delaware Bay and served as the eyes for the guns of Fort Miles at what is now Cape Henlopen State Park. As war raged in Europe, the United States moved to protect the coast from Nazi naval threats, and Fort Miles was built in two years at Cape Henlopen to defend the industrial and shipbuilding cities of Wilmington and Philadelphia, while an underwater minefield stretched from Cape Henlopen to Cape May. Thirteen observation towers were built to protect the entrance to the bay, with eleven along Delaware beaches from Fenwick Island to Cape Henlopen and two near Cape May, New Jersey. These towers were not armed; instead, observers used optical instruments such as azimuth sighting range finders to locate enemy warships, took readings every 30 seconds, and sent the coordinates to a base station bunker or gun battery, where triangulation was used to determine artillery direction. Soldiers climbed ladders through the centers of the towers, whose floors could hold four to six men and were heated in winter, and the structures were built of reinforced concrete on pilings driven into the sand. Though intended to last about 20 years and abandoned since the early 1960s, they were still standing after 60 years, and the guns of Fort Miles were never called upon to fire at an enemy ship.