Fort William and Mary was a colonial-era fortification in Great Britain's worldwide system of defenses, defended by soldiers of the Province of New Hampshire who reported directly to the royal governor. The fort, originally known as "The Castle," was situated on the island of New Castle, New Hampshire, at the mouth of the Piscataqua River estuary. It was renamed Fort William and Mary circa 1692, after the accession of the monarchs William III and Mary II to the British throne. It was captured by Patriot forces, recaptured, and later abandoned by the British in the American Revolutionary War (1775-1783). The fort was renamed Fort Constitution in 1808 following rebuilding, in light of increasing hostilities with the British again, especially from its Royal Navy, resulting in the subsequent War of 1812. The fort was further rebuilt and expanded through 1899, following the Spanish-American War. It served actively through the first half of the 20th century to World War II.

First fortified by the English Army prior to 1632, the fort guarded access to the harbor at Portsmouth. It was later manned by the successor British Army and served as the colony's main munitions depot and seaport. The fort also served to protect Kittery (then part of the Province of Massachusetts Bay colony and later in future state of Maine) on the opposite shore of the harbor, which was raided numerous times by the tribes of the Wabanaki Confederacy during the French and Indian Wars (1753-1763). Shadrach Walton, British colonial administrator and soldier, commanded the fort during different periods at the end of the 17th and beginning of the 18th century.

In 1774, it was the only permanently occupied military post in New Hampshire.

On December 14, 1774, local Patriots from the Portsmouth area, led by local political leader and rebel activist John Langdon, stormed the post (overcoming a six-man caretaker detachment) and seized the garrison's gunpowder supply, which was distributed to local militia through several New Hampshire towns for potential use in the looming struggle against Great Britain. On the following day, patriots led by colonial military officer John Sullivan again raided the fort, this time seizing with greater effort of numerous heavy artillery of cannon, ammunition and supplies for the rebel cause.

Following the Revolution, the fort was called "Castle Fort" or "Fort Castle". The new state of New Hampshire gave the ground around Fort Point, on which the old fort stood, to the federal government in 1791. In 1800, the Portsmouth Naval Shipyard was established upriver on Fernald's Island (now part of Seavey's Island), and the fort was rebuilt under the Second System of U.S. coastal defense fortifications. Brick and masonry walls were doubled in height, and new brick buildings were added inside. Work was completed in 1808 and the defense renamed "Fort Constitution". On July 4, 1809, an accidental explosion marred Independence Day / Fourth of July celebrations at the fort, killing a number of soldiers and civilians. Two years later, the U.S. Secretary of War's report on fortifications for December 1811 described Fort Constitution as "an enclosed irregular work of masonry, mounting 36 heavy guns... (with) brick barracks for two companies..." During the War of 1812 the fort was occupied and expanded with Walbach Tower, a Martello tower with a single 32-pounder cannon, being built in 1814, just before the conflict ended.

Over four decades later, during the American Civil War (1861-1865), Fort Constitution was projected to be rebuilt as a three-tiered granite fort under the new expanded, more formidable Third System of U.S. coastal defense fortifications. However, advances in weaponry, particularly the development and use of armored, steam-powered warships with heavy rifled guns, rendered the masonry walls design obsolete before they were finished. The fort's construction was abandoned in 1867 following the Civil War with the older now-obsolete Second System fort still largely intact and two walls from the revised expanded Third System cut short, built around parts of it. At some point in the Civil War era, four 100-pounder (6.4 inch, 163 mm) Parrott rifled cannon were mounted at the fort, and remained there at least four more decades through late 1903.

In 1897, construction began on Battery Farnsworth, located under the hill on which the ruins of the former Walbach Tower of 1814 stand, as part of the large-scale War Department's Board of Fortifications began the latest improvements with the Endicott Program of seacoast fortifications. It was part of the larger surrounding Coast Defenses of Portsmouth, along with nearby Fort Stark and Fort Foster in Kittery to the north. The battery was completed in 1899, a year after the successful conclusion of the Spanish-American War. Named for Union Army Brigadier General Elon J. Farnsworth (1837-1863), killed at the Battle of Gettysburg during the Civil War, the new reinforced-concrete installation, built partly below ground in a deep trench as the new protective style required, included two 8-inch (203 mm) M1888 guns on recently developed disappearing carriages. The battery was accompanied in 1904 by the construction of adjacent Battery Hackleman, named for Union General Pleasant A. Hackleman, with two 3-inch (76 mm) M1903 guns on pedestal mounts. A mine casemate for an underwater minefield in the harbor was built; Battery Hackleman was built primarily to defend this minefield against naval minesweepers vessels. These were similar to numerous other Endicott-style fortifications built all along the Eastern Seaboard and Gulf of Mexico coasts throughout the country outside of major cities on river mouths, harbors, and inlets / bays during the late 1890s and early 1900s.

After the American entry into World War I in April 1917, many guns were removed from coast defenses for shipment across the Atlantic Ocean to potential service on the Western Front in northern France, against the attacking German Empire and its Imperial German Army. Both 8-inch guns of Battery Farnsworth were removed in October 1917 for use as railway artillery and were not returned to the fort. Following the war, in 1920 a mine casemate was built next to Battery Farnsworth to replace a similar facility at nearby Fort Stark.

In the Second World War (1939/1941-1945), Battery Hackleman's 3-inch guns were sent to a new battery of the same name further south at Fort H. G. Wright on Fisher's Island, New York. They were replaced by two 3-inch (76 mm) M1902 guns taken from Battery Hays (named for Union Army General Alexander Hays, killed at the Civil War Battle of the Wilderness) at Fort Stark. In 1940-1944 the Harbor Defenses of Portsmouth were garrisoned by the 22nd Coast Artillery Regiment of the U.S. Army, and a mine observation station was built atop Battery Farnsworth. Following the surrender of the Axis Powers, Battery Hackleman was disarmed by 1948 and the fort was turned over to the Coast Guard. Battery Hackleman was eventually demolished, but Battery Farnsworth can still be seen as part of the subsequent state park.

With the site given back to the State of New Hampshire by the American military in 1961, the new Fort Constitution State Park was listed on the National Register of Historic Places in 1973 and is today open to the public.

Since 1771 the fort has been home to a lighthouse. The current installation, the Portsmouth Harbor Light, was completed in 1878. Its Fourth Order Fresnel lens remains a valuable aid to navigation. The tower and the grounds immediately around it are open for scheduled tours.

• Seacoast defense in the United States
• United States Army Coast Artillery Corps
• 8th Coast Artillery (United States)
• National Register of Historic Places listings in Rockingham County, New Hampshire
• New Hampshire Historical Marker No. 4: William & Mary Raids

• Roberts, Robert B. (1988). Encyclopedia of Historic Forts: The Military, Pioneer, and Trading Posts of the United States. New York: Macmillan. ISBN 0-02-926880-X.
• Wade, Arthur P. (2011). Artillerists and Engineers: The Beginnings of American Seacoast Fortifications, 1794–1815. CDSG Press. ISBN 978-0-9748167-2-2.
• Weaver II, John R. (2018). A Legacy in Brick and Stone: American Coastal Defense Forts of the Third System, 1816-1867, 2nd Ed. McLean, VA: Redoubt Press. ISBN 978-1-7323916-1-1.

• DeMitchell, Terri A. (2013). The Portsmouth Alarm: December 1774. Mahomet, IL: Mayhaven Publishing, Inc. ISBN 978-1932278927. (A fictionalized account of the raid on Fort William and Mary written for young adult audiences.)
• Elwin L. Page. "The King's Powder, 1774," New England Quarterly Vol. 18, No. 1 (Mar., 1945), pp. 83–92 in JSTOR
• Thomas F. Kehr, "The Seizure of his Majesty's Fort William and Mary at New Castle, New Hampshire, December 14–15, 1774," Essays and Articles, New Hampshire Society of the Sons of the American Revolution at [1]

• Fort Constitution Historic Site New Hampshire Department of Natural and Cultural Resources
• Fort Constitution from American Forts Network
• Detailed account of raids
• List of documented participants in raids
• Additional information at the University of New Hampshire
• Portsmouth Harbor Lighthouse History
• List of all US coastal forts and batteries at the Coast Defense Study Group, Inc. website
• FortWiki, lists most CONUS and Canadian forts