Prospect Hill formed an integral part of the Narragansett Bay coastal defense system from 1916 to 1945. From the crest, military observers had an unobstructed view of the approaches to the bay, the East and West passages, and mine fields laid in each during wartime. Along a 400-foot path at an elevation of 125 feet were six in-ground stations designed as observation posts and oriented generally south toward Rhode Island Sound. When the posts were in use, the surrounding landscape was entirely clear of trees, allowing clear views of the water to the east, west, and south. The posts were manned by two to four soldiers and could be reached only by ladder through an opening in the roof. Using sighting instruments, observers took bearings on targets and communicated them by telephone to plotting rooms of gun batteries at forts within the bay. Combined with information from other observation posts, this data allowed plotters to aim guns accurately at approaching enemy ships. As originally designed, positions 1, 2, and 3 served as command stations, while positions 4, 5, and 6 served as observation sites for gun batteries at Fort Wetherill and Fort Getty on Conanicut Island, Fort Greble on Dutch Island, and Fort Adams in Newport. The functions of the stations changed over time: the structure here served as the Fort Commander's Station for nearby Fort Getty in 1921, as the overall Harbor Defense Command Post in 1934, and as the seat of the Harbor Mine Command in 1943. During World War II, only positions 1 and 6 remained in active use, and personnel of the Harbor Mine Command observed both the East and West Passages while maintaining direct communication with related facilities across the island at Fort Wetherill. During that war, the 243rd Coast Artillery Regiment of the Rhode Island National Guard manned most of the coastal defense sites in the Narragansett Bay area.
