Fort William and Mary Fort William and Mary was a colonial fortification in Britain ' s worldwide system of defenses , manned by soldiers of the Province of New Hampshire who reported directly to the royal governor . The fort , known locally as " the Castle " , was situated on the island of New Castle , New Hampshire , at the mouth of the Piscataqua River estuary . First fortified by the British prior to 1632 , the fort guarded access to the harbor at Portsmouth and served as the colony ' s main munitions depot . In 1774 , it was the only permanently manned military post in New Hampshire . On December 14 , 1774 , local Patriots from the Portsmouth area , led by John Langdon , stormed the post and seized the garrison ' s powder , which was distributed through several New Hampshire towns for potential use in the looming struggle against Great Britain . On December 15 , 1774 , patriots led by John Sullivan again raided the fort , this time seizing numerous cannon . 1774 Raid . As tensions increased before the American Revolutionary War , Lord North ' s ministry became concerned that the profusion of arms in New England would lead to bloodshed . On October 19 , 1774 , King George III issued a confidential Order in Council forbidding the export of arms and powder to America . Word of the order reached operatives in New England ' s patriot movement . The port at Boston had been closed in punishment for the Boston Tea Party , and the Portsmouth Committee of Correspondence kept in close contact with friends of liberty in Boston . Tensions in Massachusetts nearly erupted into violence in the fall of 1774 when redcoats seized provincial gunpowder during the so - called Powder Alarm . Upon learning of the Order in Council , patriots feared that the British military would make another attempt to seize colonial stores . Patriots in Rhode Island moved munitions from the fort at Newport inland for safe keeping without incident . In Massachusetts , rumors flew that troops from Boston were headed to reinforce Fort William and Mary and seize its powder and arms . On December 13 , 1774 , four months before his more famous ride in Massachusetts , Paul Revere rode to Portsmouth to sound the alarm . On the morning of December 14 , Patriots from the town of New Castle unsuccessfully attempted to take the gunpowder at Fort William and Mary by trickery . Meanwhile , John Langdon made his way through Portsmouth with a drummer , collecting a crowd to descend on the fort . Several hundred men responded to his call , setting out for the Castle by way of the Piscataqua River . Only one provincial officer , Captain John Cochran , and five provincial soldiers were stationed at Fort William and Mary . Despite the odds against them , they refused to capitulate to Patriot demands . When Langdon ' s men rushed the fort , the defenders opened fire with three cannon and a volley of musket shot . Patriots stormed the walls and Cochran ' s men engaged in hand - to - hand fighting before being subdued by an overwhelming number of raiders . Langdon ' s volunteers not only broke open the powder house and absconded with about 100 barrels of gunpowder but , to three cheers , hauled down the fort ' s huge British flag . Several injuries but no deaths occurred in the engagement , and Cochran and his men were released after about an hour and a half of confinement . The next day , additional rebel forces arrived in Portsmouth from across the colony , as well as from Maine . Led by John Sullivan , the rebels returned to the fort late on the night of December 15 , overran the post without gunfire and removed muskets , military supplies and 16 cannon marked as the property of the King . British authorities declared the raids - for which Sullivan later received a stipend from the Continental Congress - high treason . Aftermath . In response to a call for aid from Boston by Governor John Wentworth , the armed hydrographic survey sloop " Canceaux " arrived to keep the peace in New Hampshire on December 17 , followed by the twenty - gun frigate " Scarborough " on December 19 , with numerous marines aboard . The Governor and his family were driven from their home in Portsmouth in the summer of 1775 and forced to take refuge in the fort , guarded by the guns of British warships . Britain finally gave up on the colony of New Hampshire in order to focus attention on the military situation in Massachusetts and abandoned the fort , removing its remaining equipment to Boston along with Governor Wentworth . The supplies captured by Patriots in December of 1774 were later used by New Hampshire ' s forces against the British military , including in the Siege of Boston . Conversely , supplies ( including numerous cannon ) left in the fort by Patriots following the raids were subsequently put to use by the British forces . Fort Constitution . The state gave Fort Point , on which Fort William and Mary 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 . 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 . During the War of 1812 the fort was manned and expanded , Walbach Tower , a Martello tower , being built in 1814 . During the Civil War , Fort Constitution was rebuilt as a three - tiered granite fortress . However , advances in weaponry , particularly armored , steam - powered warships with heavy guns , rendered the masonry design obsolete before it was finished . In 1897 , construction began on Battery Farnsworth , located under the hill on which Walbach Tower stands . Named for Brigadier General Elon J . Farnsworth , the installation included two 8 - inch breech - loading rifle s on disappearing carriages . 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 . The lighthouse . 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 .