In the 1780s, the French government launched a "Hudson Bay Expedition" to damage HBC activities in that bay. Three French warships of the Expedition, led by Jean-François de La Pérouse, captured the Prince of Wales Fort in 1782. The fort was manned by only 39 (non-military) men at the time, and the fort's Governor, Samuel Hearne, recognised the numerical and military imbalance and surrendered without a single shot being fired.
Let's give the HBC a year's notice of the August 1782 attack. What size garrison could the fort maintain? How would you set up the defence?