The concrete towers along the coast of Delaware are WWII observation towers. There’s one that’s open to the public at Cape Henlopen State Park near Rehoboth.
At the begining of the war, the U.S. was pretty much defenseless against U-boat attacks. This should have been anticipated with our WWI experience and Britain’s experience since 1939 but too many Americans felt that isolation (and a low defense budget) would keep us safe. What anti-sub capabilities we had were used to protect convoys crossing the Atlantic.
German subs had a field day off the east coast, lying in wait for the inevitable oil tanker/cargo ship and then surfacing to recharge their batteries. A number of coastal residents claimed that they could see German sailors sunbathing on the deck. There were so few anti-submarine boats or planes that the odds were stacked on the German side.
A lot of measures were instituted to counter the threat. The concrete towers were built to radio sightings to the closest plane/ship. Coastal fortifications were upgraded - including historical sites like Ft. Sumter in Charleston, SC. Anti-sub ships and planes were built and rushed into service at new coastal airfields, and patrols increased. Cargo was shifted to a strained rail system. An interstate pipeline system was designed and built to move Gulf Coast oil to the Northeast.
Evetually the odds turned, the anti-sub web of planes and ships extended far into the Atlantic, and the U.S. coast became a death trap for German subs.
I’m not sure that the observation towers ever got a kill - the Germans probably noted their locations and moved away. After the war, the towers, the airfields, and most of the forts moldered away. The pipelines proved so economical that they were expanded to ship natural gas from the Gulf in the late 1940’s. And the submarine threat (this time Russian) to cargo was one of the reasons for the Eisenhower administration to build the interstate highway system.