Castle Williams is a circular defensive work of red sandstone on the west point of Governors Island in New York Harbor. It was designed and erected between 1807 and 1811, designed by the Chief Engineer of the US Army Corps of Engineers, Lt. Col. Jonathan Williams, , and considered a prototype for new forms of coastal fortification. The castle was one component of a larger defensive system for the inner harbor that included Fort Jay and the South Battery on Governors Island, at the tip of Manhattan, Fort Gibson at (then Oyster Island), and Fort Wood, which is now the base of at Liberty Island (then Bedloe's Island). This system of forts came to be known as the Second American System of coastal defense and existed to protect harbors like the one in New York from British interference with American Shipping.
Its usefulness as a fort began to end in the 1830s, so Castle Williams subsequently served as barracks for the island's garrison and new and transient troops. Thereafter, the castle was remodeled by the US Army for use as a in various forms during the Civil War and through the first half of the 20th century.
