Great Gull Island was originally built to protect the US from British invasion, in 1894. Its attention was turned toward Germany for the world wars. It supported disappearing carriage gun placements. This meant the gun would sink back and down on its own recoil. It would be hoisted back up to firing position by counterweights located in a pit. The largest gun, 16 inches, fired rounds weighing 2100 pounds.

After becoming antiquated it was sold to the Museum of Natural History, for one dollar. Helen Hays, from the museum has been running a research project to study Terns. Great Gull Island is one of the few places they nest.

The Gun Placements are still there, though the actual guns were removed. To maintain the guns, as well as store ammunition, a large buried series of caverns was constructed. One of them sports a car from a salvage operation, another was constructed of bright pink bricks, used to absorb moisture. The largest battery still has rail tracks into its underbelly. This was used to deliver ammunition brought in from the pier.

At one time the island had a lot more buildings, but they were destroyed in the hurricane of 1938. The ruins of these buildings can be seen strewn along the island's shoreline There was even a hospital among the long gone buildings.

The Island still has a few barrack buildings that are sometimes used as dorms for interns, and volunteers for the Tern research project. The island also sports fire control towers, which were used for target spotting. One has a stairwell leading down to a narrow hidden corridor. This leads to the underbelly of the larger battery.