Castle Williams is a historic fortification located on Governors Island in New York Harbor. Built between 1807 and 1811, it was designed by architect John McComb Jr. as part of the city's coastal defense system. The structure features a distinctive circular design and was named after Colonel Jonathan Williams, a key figure in early American military engineering. The fort played a significant role during the War of 1812 and later served various military purposes until it was decommissioned in the 1960s. Today, Castle Williams is part of the Governors Island National Monument and is open to the public for tours and educational programs.