Near the end of the Civil War, several large cannon were mounted on the ravelin above and in the adjacent water battery, and the commanding officer ordered that 1,000 rounds of ammunition be kept nearby for each gun. In 1866, this magazine and two others were built to safely store the additional gunpowder. The covered entrance with its angled stairway was designed to prevent enemy projectiles from reaching the explosives, and three feet of concrete and up to 16 feet of earth were laid over the arched brick vault. In addition to the underground magazines, Fort McHenry had two above-ground brick magazines, one inside the star fort and one 250 yards to the west. Gunners took care to keep their powder dry and away from flames, and powder kegs were probably stored on wooden racks where they could be periodically rotated to prevent caking. Ferrous metal objects that could produce sparks were prohibited in the magazine.