Fort Longueuil was recognized as a National Historic Site of Canada on May 25, 1923. The historic site includes the archaeological site of the fort, which was demolished in 1810. The site extends beneath the present-day Saint-Antoine-de-Padoue Cathedral.
The territory of New France was divided into seigneuries in order to ensure the colony's defence. Of these seigneuries, Charles Le Moyne was granted the Seigneury of Longueuil. His son, Charles Le Moyne de Longueuil, built a fort with four towers between 1685 and 1690 as his fortified residence. The fort was built entirely in stone masonry, and included a guard house, a chapel and corps de logis.
Louis de Buade de Frontenac and Jean Bochart de Champigny then asked King Louis XIV to erect the seigneury of Longueuil as a barony, which he did in 1700, establishing the Barony of Longueuil. Charles Le Moyne de Longueuil was the only Canadian-born person to be raised to the rank of Baron by a French King.
Fort Longueuil was believed to be occupied by American troops during the American Revolutionary War. It was subsequently occupied by the British. It was demolished in 1810 because of its poor condition.
The fort was located in the Old Longueuil neighbourhood. It was partly situated on the site of the Co-Cathedral of Saint-Antoine-de-Padoue, Saint-Charles Street, Chambly Road, the site of a Laurentian Bank branch and a restaurant.
It measured 68 meters long by 46 meters wide. The interior of the fort included:
- a house 72 X 24 feet
- a chapel of 45 X 22 feet
- a guard house of 25 X 25 feet
- a barn of 70 X 30 feet
- a barn to house 12 horses
- stables to house 40 cattle
- a barn 30 feet squared and other necessary farm buildings
- Fort Longueuil. Canadian Register of Historic Places. Retrieved 7 April 2012.
- Acte de concession du comte de Frontenac et de Jean Bochart de Champigny, gouverneur et intendant de la Nouvelle-France, à Charles Le Moyne, écuyer, sieur de Longueuil, le 25 septembre 1698, publié dans Pièces et documents relatifs à la tenure seigneuriale, demandés par une adresse de l'Assemblée législative, 1851, E. R. Fréchette, Québec, 1852, 484 p., p. 445-6.
- Louis Lemoine, Le château fort de Longueuil (1698-1810), Société d'histoire de Longueuil, 1987, p. 21
- Aveu du 17 avril 1723, in Louis Lemoine op. cit. p. 60-61
- Alex Jodoin et J. L. Vincent, Histoire de Longueuil et de la famille de Longueuil, avec gravures et plans, Montréal, Imprimerie Gebhart-Berthiaume, 1889, 681 p.
- Louis Lemoine, Le château fort de Longueuil (1698-1810), Société d'histoire de Longueuil, 1987, 152 p.