Following the Revolution the fort was called Castle Fort or Fort Castle.
The state gave Fort Point, on which the 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 US fortifications.
Walls were doubled in height and new brick buildings added.
Work was completed in 1808 and the defense renamed "Fort Constitution".
On July 4, 1809, an accidental explosion marred Independence Day celebrations at the fort, killing a number of soldiers and civilians.
The Secretary of War's report on fortifications for December 1811 describes 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 manned and expanded, Walbach Tower, a Martello tower with a single 32-pounder cannon, being built in 1814.

During the Civil War, Fort Constitution was projected to be rebuilt as a three-tiered granite fort under the Third System of US fortifications.
 However, advances in weaponry, particularly armored, steam-powered warships with heavy rifled guns, rendered the masonry design obsolete before it was finished.
The fort's construction was abandoned in 1867 with the Second System fort largely intact and two walls from the Third System built around parts of it.
At some point in the Civil War era, four 100-pounder (6.4 inch, 163 mm) Parrott rifles were mounted at the fort, and remained there at least through late 1903.

In 1897 construction began on Battery Farnsworth, located under the hill on which the ruins of Walbach Tower stand, as part of the large-scale Endicott Program of seacoast fortifications.
It was part of the Coast Defenses of Portsmouth, along with Fort Stark and Fort Foster.
The battery was completed in 1899.
Named for Brigadier General Elon J. Farnsworth, the installation included two 8-inch (203 mm) M1888 guns on disappearing carriages.
The battery was accompanied in 1904 by Battery 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 minesweepers.

After the American entry into World War I in early 1917, many guns were removed from coast defenses for potential service on the Western Front.
 Both 8-inch guns of Battery Farnsworth were removed for use as railway artillery in October 1917 and were not returned to the fort.
In 1920 a mine casemate was built next to Battery Farnsworth to replace a similar facility at Fort Stark.

In World War II Battery Hackleman's 3-inch guns were sent to a new battery of the same name at Fort H. G. Wright on Fisher's Island, New York.
They were replaced by two 3-inch (76 mm) M1902 guns from Battery Hays at nearby Fort Stark.
In 1940-1944 the Harbor Defenses of Portsmouth were garrisoned by the 22nd Coast Artillery Regiment.
Also, a mine observation station was built atop Battery Farnsworth.
Battery Hackleman was disarmed by 1948 and the fort was turned over to the Coast Guard.
Battery Hackleman was demolished, but Battery Farnsworth can still be seen.

Given back to the state in 1961, Fort Constitution State Park was listed on the National Register of Historic Places in 1973, and is today open to the public.
Following the Revolution the fort was called Castle Fort or Fort Castle. The state gave Fort Point, on which the 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 US fortifications. Walls were doubled in height and new brick buildings added. Work was completed in 1808 and the defense renamed "Fort Constitution". On July 4, 1809, an accidental explosion marred Independence Day celebrations at the fort, killing a number of soldiers and civilians. The Secretary of War's report on fortifications for December 1811 describes 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 manned and expanded, Walbach Tower, a Martello tower with a single 32-pounder cannon, being built in 1814.
During the Civil War, Fort Constitution was projected to be rebuilt as a three-tiered granite fort under the Third System of US fortifications.  However, advances in weaponry, particularly armored, steam-powered warships with heavy rifled guns, rendered the masonry design obsolete before it was finished. The fort's construction was abandoned in 1867 with the Second System fort largely intact and two walls from the Third System built around parts of it. At some point in the Civil War era, four 100-pounder (6.4 inch, 163 mm) Parrott rifles were mounted at the fort, and remained there at least through late 1903.
In 1897 construction began on Battery Farnsworth, located under the hill on which the ruins of Walbach Tower stand, as part of the large-scale Endicott Program of seacoast fortifications. It was part of the Coast Defenses of Portsmouth, along with Fort Stark and Fort Foster. The battery was completed in 1899. Named for Brigadier General Elon J. Farnsworth, the installation included two 8-inch (203 mm) M1888 guns on disappearing carriages. The battery was accompanied in 1904 by Battery 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 minesweepers.
After the American entry into World War I in early 1917, many guns were removed from coast defenses for potential service on the Western Front.  Both 8-inch guns of Battery Farnsworth were removed for use as railway artillery in October 1917 and were not returned to the fort. In 1920 a mine casemate was built next to Battery Farnsworth to replace a similar facility at Fort Stark.
In World War II Battery Hackleman's 3-inch guns were sent to a new battery of the same name at Fort H. G. Wright on Fisher's Island, New York. They were replaced by two 3-inch (76 mm) M1902 guns from Battery Hays at nearby Fort Stark. In 1940-1944 the Harbor Defenses of Portsmouth were garrisoned by the 22nd Coast Artillery Regiment. Also, a mine observation station was built atop Battery Farnsworth. Battery Hackleman was disarmed by 1948 and the fort was turned over to the Coast Guard. Battery Hackleman was demolished, but Battery Farnsworth can still be seen.
Given back to the state in 1961, Fort Constitution State Park was listed on the National Register of Historic Places in 1973, and is today open to the public.
Fort William and Mary