Initially completed in 1859, it was also used for mustering and training recruits and new units for the Civil War from 1861 and began secondary use as a long-term military prison in 1868.

Major improvements, needed because of changes in artillery, began in the 1870s, but were underfunded and never fully completed, as the post came to be seen less as defense and more as a jail.

Later in 1846, acting in his capacity as Military Governor of California, John C. Fremont, champion of Manifest Destiny and leader of the Bear Flag Republic, bought the island for $5,000 in the name of the United States government from Francis Temple.

[1][2] In 1850, President Millard Fillmore ordered that Alcatraz Island be set aside specifically for military purposes based upon the U.S. acquisition of California from Mexico following the Mexican–American War.

The upper part of the island was encircled with walls built of stone and bricks abutting the rock faces, unlike most other Third System works, which laid a heavier hand on the natural landscape.

Army engineer James B. McPherson, later to become a prominent general during the Civil War, was one of the earliest commanders to serve on Alcatraz in 1858.

Starting in 1863, the military also held private citizens accused of treason, after the writ of habeas corpus in the United States was suspended.

For example, in 1863, Asbury Harpending outfitted a schooner, the J. M. Chapman as a confederate privateer, intending it to prey on the Pacific Mail ships, which often carried cargoes of precious metals.

[9] In the 1870s, Major George Mendell ordered the prisoners, aided by mules, to assist in changing the natural landscape of the island and creating a top level, dumping debris into the coves and bay.

[6] Between 1873 and 1895, 32 Native Americans were imprisoned at the citadel on Alcatraz, including 19 Hopi men held in captivity there between January and August 1895 after being transferred from Fort Defiance.

[6] The island continued to develop in the 1880s and in 1898 the population of Alcatraz increased dramatically from 26 to over 450 due to the Spanish–American War and placed a demand for new buildings.

This building was modernized in 1934 when it reopened as a federal penitentiary, the notorious Alcatraz prison which held America's most ruthless criminals until 1963.

[11] Modernization of the prison began in October 1933 and Alcatraz Federal Penitentiary opened in August 1934, ending some eighty years of U.S. Army occupation.

Upon embarking at the wharf, the citadel building was reached by going up through the sloping passage behind the sally port, continuing to the top of the island, and crossing a drawbridge over the 12 feet (3.7 m) deep dry moat.

[6] Battery McClellan was equipped with a 15-inch Rodman gun weighing over 25 tons, capable of launching a 330-pound explosive 4,680 yards when angled at 25 degrees.