Often referred to as the Gunpowder Store. The engine house was used by the Caledonian Canal Company during the building of the lower lock in Fort Augustus.
The Engine house was the location of the steam engine pump which pumped out the waters of Loch Ness, from the lower lock during its construction.
Whether the fort used the engine house as a gunpowder store is unknown, the location is sufficiently distant from the fort and if it does pre date the canal, it would be on the rivers shoreline. However it would be on the edge of the town which grew up.