This building was designed to store gunpowder in barrels. The magazine contains three barrel-vaulted storage chambers; shown externally by the three separate rooflines and gables.

Before electricity became available, lighted lamps (either candle or oil) were used to light the chambers at night. However, fire was a considerable threat to a magazine; it would take just one spark… Clever design improved safety. Using a perimeter internal passageway that runs around and between the internal vaults; incorporating small, glazed windows at five metre intervals, the lighted lamps were placed on the exterior sills to illuminate the interior of the chambers.