By the late 1850's, the first military prisoners were being housed on the island. While the defensive necessity of Alcatraz diminished over time (the island never fired its guns in battle), its role as a prison would continue for more than 100 years. In 1909, the Army tore down the Citadel, leaving its basement level to serve as the foundation for a new military prison. From 1909 through 1911, the military prisoners on Alcatraz built the new prison, which was designated the Pacific Branch, U.S. Disciplinary Barracks for the U.S. Army. It was this prison building that later became famous as "The Rock."