 Washington became a popular place for freed slaves to congregate, and many were employed in constructing the ring of fortresses that eventually surrounded the city.At the beginning of the war, Washington's only defense was one old fort (Fort Washington, away to the south), and the Union Army soldiers themselves.When Maj. Gen. George B. McClellan assumed command of the Department of the Potomac on August 17, 1861, he became responsible for the capital's defense.