The original dock was completed in 1854 and has been modified and enlarged several times since then. The large four-story building immediately behind the dock was once a military barracks, with its ground floor built between 1865 and 1867. Designed to house soldiers and cannon to defend the dock, its ten-foot-thick brick walls made it virtually bomb-proof. Along the dock, a row of brick gun casements can still be seen. The restored guard tower is a remnant of the federal penitentiary era, when six towers around Alcatraz were manned by armed guards whose clear view made escape by prisoners extremely difficult.