The long gun magazine, which is located on the western side of the officers new barracks, at the southern end of the Castle complex, was originally constructed in the late 18th century as a simple rectangular magazine with a small entrance lobby on its southern side. In 1888-1889 changes were made to the magazine for the storage of shells for coastal artillery. The roof was recovered with concrete and asphalt and tunnels were added to the north and south of the rectangular magazine. The structure was covered with earth banking and a blast bank was added to the south, presumably for added blast protection. (location accurate to the nearest 1m based on available information)