In 1850, a presidential order set aside the island for possible use as a United States military reservation. The California Gold Rush, the resulting boom in the growth of San Francisco, and the need to protect San Francisco Bay led the U.S. Army to build a Citadel, or fortress, at the top of the island in the early 1850's. The Army also made plans to install more than 100 cannon on the island, making Alcatraz the most heavily fortified military site on the West Coast. Together with Fort Point and Lime Point, Alcatraz formed a "triangle of defense" designed to protect the entrance to the bay. The island was also the site of the first operational lighthouse on the West Coast of the United States.