The story might have ended there, an amusing footnote in the annals of military surveying errors. But in 1842, the Webster-Ashburton Treaty redrew the boundary, and the disputed point at Rouses Point returned to American territory. Two years later, construction began on a second fortification on the same spot, this one properly named Fort Montgomery. Built between 1844 and 1871, the new fort was one of the rare "Third System" or "Permanent" forts constructed along the Northern Frontier rather than the Atlantic Coast. Over a roughly thirty-year construction period, no expense was spared. At peak activity, some 400 stonecutters and masons worked the site, shaping the massive stone walls that would rise to imposing height with gun emplacements for 125 cannons arranged across three tiers.

Fort Montgomery was one of only nine forts in the United States constructed with a moat, sharing that distinction with Fort Jefferson in Florida's Dry Tortugas. The moat, combined with Lake Champlain itself, meant the fortress was essentially an island accessible only by retractable drawbridge. The drawbridge mechanism was an engineering marvel of its era: it pivoted on a central balance point like a seesaw, one end rising to block the entrance while the other dropped into a curved pit in the postern behind the doorway. Raising it left the fort's entrance suspended high above the water of the moat below, cutting off all access by land. A second drawbridge on the lake side -- a "water gate" -- connected to a dock extending into Champlain. Behind the fort, between its walls and the shoreline, engineers constructed a manmade earthen berm called the "cover face," standing higher than the fort itself to shield it from siege artillery on land.

Construction accelerated to a frenzied pace during the American Civil War, fueled by rumors that Britain might intervene against the Union from Canada. A detachment of the 14th U.S. Infantry garrisoned the fort for three months in 1862 to signal American resolve. Those fears proved less far-fetched than they might have seemed: in 1864, twenty-one Confederate soldiers launched the St. Albans Raid from Canadian territory into nearby Vermont, the northernmost military action of the Civil War. During this period, Chief Engineer of the U.S. Army Joseph Totten invented an iron-reinforced embrasure for cannon to better protect gunners inside a fort, and this innovation was retrofitted into Fort Montgomery's upper gun tier while the lower level retained its original brick embrasures.

Fort Montgomery was designed to hold 800 men, yet it was never fully garrisoned. Like many Third System forts, it served primarily as a deterrent -- a massive stone warning aimed squarely northward. By 1886, its peak armament year, the fort mounted 74 of its 125 cannon capacity, including Rodman cannons, most pointed toward Canada. Two additional large Rodmans sat on the parade ground for years, waiting to be hoisted atop the walls but never installed. In 1880, Commanding General of the Army William Tecumseh Sherman toured the site, was so impressed by its scale that he wanted to relocate the nearby Plattsburgh Barracks garrison to the fort. Local citizens protested, and the move never happened. As the century turned and explosive shells and rapid-firing rifled artillery rendered masonry forts obsolete, the guns were slowly removed. By 1901, only twenty remained.

Fort Montgomery's later history reads like a dark comedy. The military abandoned it, and the property eventually passed into private hands. In the mid-1980s, the owner worked with local historical societies to have New York State purchase the fort for possible restoration. The state declined -- even when offered the property for free. In May 2006, the owner's heirs listed the fort on eBay. The auction closed on June 5 with a winning bid of $5,000,310, but the sale fell through. The fort remains unsold. Meanwhile, preservationists warn that the ruins face catastrophic structural collapse. Iron reinforcing rods installed around 1886 to support the massive outer wall were likely cut out for scrap during World War II metal drives, undermining the fort's structural integrity. A third bastion on the northern side already collapsed completely in 1980, tumbling into the moat. In 2008, the Preservation League of New York State named Fort Montgomery one of its "Seven to Save" properties -- a recognition of both its historical importance and its precarious future.