Part of the metropolitan park system, Castle Island is said to be the site of the oldest continuously used military fortification in the U.S. -- it was built by the British in the 1600s and came into American hands in 1776. Castle Island was also the home of the Commonwealth's first prison and the nation's first marine hospital, both opened in 1789. It was also the military post of Edgar Allan Poe when he served in Battery H of the First Artillery in 1827.