Castle Williams is a circular defensive work of red sandstone on the west point of Governors Island in New York Harbor, designed and erected between 1807 and 1811.

It was designed by the Chief Engineer of the US Army Corps of Engineers, Lt. Col. Jonathan Williams, for whom the fort is named, and considered a prototype for new forms of coastal fortification.

he castle was one component of a larger defensive system for the inner harbor that included Fort Jay and the South Battery on Governors Island, Castle Clinton at the tip of Manhattan, Fort Gibson at Ellis Island (then Oyster Island), and Fort Wood, which is now the base of Liberty Enlightening the World 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.

It remains open to the public.