Castle Williams is a circular fort made of red sandstone on the west point of Governors Island in New York Harbor. It was built between 1807 and 1811, as part of a defense system for New York 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. This system protected the harbor from British interference with American shipping. It later served as barracks for the island's garrison and new and transient troops, and later as a prison during the Civil War and through the first half of the 20th century. Castle Williams is open to the public. A replica of the castle is on display in the middle of the fort. Guided tours are also available.