Water colour showing the interior courtyard, the officers' quarters and the guardhouse.
Fort Lennox, Isle aux noix
© Royal Ontario Museum, Toronto / J.D.MacDiarmid / 958.2604, circa 1838

The War of 1812 resulted in a re-evaluation of the strategic role to be played by île aux Noix. The new road between Missisquoi Bay and Saint-Jean reduced the importance of the island for the region's defence. Nevertheless, the colonial leaders decided to erect a major fortification in reaction to a Fort, which the Americans had begun building at Rouses Point, less than 15 km south of the island.

Construction lasted from 1819 to 1829, and was carried out in accordance with plans drawn up by engineer Nicolls. First, a rectangular earth enceinte with bastions at the corners was built, along with fosse, redan and ravelin. Following this, several other structures in a neoclassical style were added: the powder magazine, two warehouses (ordnance magazine and artillery magazine), a guardhouse, an officers' quarters, a barracks and 17 casemates. The new fort was named Lennox, in honour of Charles Lennox, Duke of Richmond, who died in 1819 during his term as Governor-in-Chief of British North America.