Castle Clinton National Monument. Last of a series of forts which from the Dutch settlement of 1624 guarded lower Manhattan. This structure was built by the United States in the years 1808 to 1811. It was first called "West Battery" and was one of the important defenses of New York Harbor during the War of 1812 period . Named in honor of Gov. DeWitt Clinton in 1815. It was made headquarters of the U.S. Third Military District from 1816 to 1820. Gen. Winfield Scott was in command. The Headquarters was removed from Castle Clinton to Governor's Island in 1820