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 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).

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.

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..."[5] 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.

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.

[4] 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.

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.

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.

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.