During the 20 years Fort Knox was under construction, changing plans forced some sections to be rebuilt, including Battery B. Built in 1845 for 15 guns, Battery B was the first part ready for cannons, but none arrived. Battery B and Battery A, 530 feet to the south, still had no cannons when the Civil War began in 1861. After Maine's government pressed the federal Department of War to arm Fort Knox and other Maine forts, the federal government responded, but the original emplacements in Batteries A and B had been built for 32-pounder cannons, which had been superseded by larger and more powerful 10-inch Rodman cannons. As a result, crews working for Capt. Thomas Lincoln Casey, the supervising engineer in 1861, spent months rebuilding and reinforcing the gun emplacements. The first cannons installed there, two 10-inch Rodman cannons, arrived in 1863. Over the next three years, six 10-inch Rodmans and an even larger 15-inch Rodman were added. The single cannon barrel remaining there is from a 15-inch Rodman and was one of the cannon parts saved from World War II salvage efforts and left there for educational purposes.