Fort Augustus is at the south-west end of Loch Ness, at a key junction of the military roads constructed after 1715; built in 1729, the fort replaced a barracks known as Kilwhimen Barracks. Instead of being built on high ground, it was placed on a peninsula surrounded by Loch Ness and the Rivers Oich and Tarff. Cumberland felt it could not be defended for more than a few days, and wrote that Fort William was the only one of any importance.

Square in plan with angled bastions on each corner, the fort was designed "more ... for ornament than strength", as a demonstration of the government presence in the Scottish Highlands. The walls were weak, while the six-pounder guns that provided defensive fire were installed on top of the four bastions, in full view of an attacking force. The garrison consisted of three companies from Guise's Regiment, commanded by Major Hugh Wentworth. He lacked trained gunners and stationed one of his companies in the old Kiliwhimen Barracks, an isolated position to the south of the fort. This was quickly taken by the French regulars under Stapleton, and his engineer Grant began siege operations on 22 February 1746.

According to an eye witness, the Jacobites had three batteries, one opposite the main gate and two firing from the north. These had little effect, with most of the damage being done by three coehorn mortars; on the first day, a shell from one of these blew up the fort's magazine, destroying one of its bastions. A second shot caused the explosion of another magazine the next day, but firing then continued for another four days without further impact. The garrison capitulated on 1 March without any casualties, and Wentworth was considered to have surrendered too early; he was subsequently court-martialled and dismissed from the army.