Once called West Battery, the castle used to be a fort that was meant to protect the city from the attacks from the sea.
Later on, it served many different purposes (for a while it was even an aquarium). Nowadays, most tourists visit it in order to get their tickets to the Statue of Liberty and Ellis Island which are sold there.
However, Castle Clinton is a place with interesting history. It was here where the first immigration station was placed (before the Ellis Island began to serve as an immigration station in 1892). More than 8 million immigrants went through this station on their way to the US.
There is no entrance fee and the castle is located in the beautiful Battery Park, which is definitely worth visiting.
You can get here by subway line 1 (get off at South Ferry/Whitehall) or by lines 4 and 5 to Bowling Green.
The admission is free.
Official website http://www.nps.gov/cacl/
Phone +1 212 344 7220
Address 10 Battery Place, 10004
Coordinates 40°42'12.448" N -74°1'0.382" E