The first important structure built on Alcatraz was the lighthouse, in the mid-1800s. This was the first lighthouse built on the American West Coast. In 1850 it became a military fort, and later a military prison. It housed prisoners and Southern sympathizers during the Civil War, and it went on to house Spanish-American war prisoners, and conscientious objectors to World War I. Side Note: Kudos to this photographer for framing the cruise ship and Alcatraz with the window; they look like they are about the same size in this shot. Where would you rather be? Personally, for me it's kind of a toss up.