This is the magazin general — a sort of general store for the 18th century — at the Fortress of Louisbourg. In fact I’m pretty sure it directly translates as general store. It’s a giant stone building on the main street of the fortress, directly across from the water. I can imagine a couple of centuries ago the workers unloading supply boats and wheeling or dragging the supplies across the dusty street to the store to be stored and sold. Judging by the thickness of the walls and the darkness of the interior it probably kept the more perishable items at a reasonable temperature.
There are a lot of interesting buildings at the fortress, definitely an interesting place to spend a day wandering.