Analysis of information sources in references of the Wikipedia article "Alcatraz Island" in English language version.
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the army continued to work on Alcatraz throughout 1860 and 1861, expanding and improving the island's existing fortifications. The military also used the island as a training ground for soldiers. New troops continually arrived on the island, underwent training, and departed for other assignments. With many new enlistees, the military personnel on Alcatraz increased to over 350 by the end of April 1861. The army slowly increased the number of men assigned to Alcatraz throughout the Civil War, reaching a high point of 433 men in early 1865. The army shipped most of these soldiers out to the Southwestern frontier; however, some were sent to battlefields in the East.
Alcatraz is less known in its former military role as a 19th-century coastal fortification protecting the interests of a rapidly westward-expanding nation during the turbulent era of Manifest Destiny, the 1849 Gold Rush and the Civil War. The fortification, with its underground ammunition magazines and tunnels, is important from a military history perspective, marking the transition to earthen structures from the traditional brick and masonry constructions that characterized earlier 19th-century coastal defences.