The Guardhouse, one of the oldest and most significant structures on Alcatraz Island, housed the island's very first military prison. Shortly after California became a state, the U.S. Army took possession of Alcatraz and quickly recognized that the island's rocky cliffs and nearly inaccessible shoreline offered excellent defensive features. The army constructed a ring of gun batteries circling the island and in 1857, the army built the Guardhouse with a 'sally port,' which is a secured, controlled entryway to the fort.