Powder magazines are structures designed to protect gunpowder and ammunition from moisture, sparks, and impact. In 1814, a much smaller magazine stood here. During the famous bombardment, a British shell crashed into the roof but miraculously failed to set off an explosion. Immediately after the battle, work began on a stronger magazine, the one that stands today. Its brick walls and arched roof are so thick that the space inside accounts for only 10 percent of the building's total volume.