Fort Knox, now Fort Knox State Park or Fort Knox State Historic Site, is located on the western bank of the Penobscot River in the town of Prospect, Maine, about 5 miles (8.0 km) from the mouth of the river. Built between 1844 and 1869, it was the first fort in Maine built entirely of granite; most previous forts used wood, earth, and stone. It is named after Major General Henry Knox, the first U.S. Secretary of War and Commander of Artillery during the American Revolutionary War, who at the end of his life lived not far away in Thomaston. As a virtually intact example of a mid-19th century granite coastal fortification, it was added to the National Register of Historic Places in 1969 and declared a National Historic Landmark on December 30, 1970. Fort Knox also serves as the entry site for the observation tower of the Penobscot Narrows Bridge that opened to the public in 2007.
Construction began in 1844 and continued until all masonry fort funding was withdrawn in 1869, with the fort mostly complete except for the emplacements on the "roof" or barbette level. Funding from Congress was intermittent, and the fort's design was never fully completed despite an expenditure of $1,000,000. Granite was quarried five miles (8 km) upriver from Mount Waldo in Frankfort. The fort's overall design was by Joseph G. Totten, the foremost fortification engineer of the Army Corps of Engineers in his day. Notable engineer officers supervising construction included Isaac Ingalls Stevens and Thomas L. Casey.
Besides the main fort with 64 guns, Fort Knox had two open water batteries facing the river, each equipped with a shot furnace to heat cannonballs sufficiently that they could ignite wooden ships if the ball lodged in the vessel. These furnaces became obsolete with the adoption of ironclad warships.
Civil War
Fort Knox never saw battle, though it was manned during times of war. During the American Civil War volunteers from Maine, mostly recruits in training before assignment to active duty, manned the fort. Thomas Lincoln Casey supervised work on the fort, including adapting the batteries to use the recently invented Rodman cannon, and oversaw its completion.
Spanish-American War
A regiment from Connecticut manned Fort Knox during the Spanish-American War. A plaque at the fort describes the laying of a controlled minefield in the river during this war, which Congress appropriated $3,200 for shortly after its outbreak.
Post Spanish-American War
The garrison was reduced to one man, the "Keeper of the Fort" or caretaker with the rank of ordnance sergeant, at the end of the war. The keeper attended to the condition and maintenance of the fort, and reported to Fort Preble in South Portland. In 1900 the fort received a permanent "torpedo storehouse" for storing naval mines (which were called torpedoes at the time) that is now the Visitor's Center.
In 1923, the federal government declared the fort excess property and put its 125-acre (51 ha) grounds up for sale. The state of Maine bought it for $2,121. It has been administered as a Maine state historic site since 1943.