Castle Clinton stands where New York City began,” writes the National Park Service website. Located at the southern tip of Manhattan, the sandstone fortification — also known as Fort Clinton — was initially one of four constructed forts intended to prevent a British invasion in 1812. However, its reputation is cemented as America’s first immigration station. Prior to Ellis Island, it welcomed more than 8 million people arriving to the United States between the years 1855 to 1890, and it’s been repurposed as a beer garden, theater and public aquarium over the course of its life.

Today, the National Monument continues to welcome visitors to New York Harbor. As the departure point for guests who join us for our hard hat tour of the abandoned hospital complex at Ellis Island (which you purchase tickets for below), Castle Clinton has continuously peaked our interest. You may also have seen it prominently featured recently in the new show The Alienist.

Here, we delve into 10 of its most interesting secrets and fun facts, covering its two century long lifespan:

Behind-the-Scenes Hard Hat Tour of the Abandoned Ellis Island Hospital

1. Castle Clinton Was Once Its Own Man-Made Island

Castle Clinton on top left, as an island as mapped in the Commissioner’s Grid of 1811. Original map is located in the Library of Congress, photographed by Untapped Cities above.

According to an 1808 original plan for Castle Clinton, the fortification was originally built on a landfill isle that was constructed on a stone foundation. A 200-foot wooden causeway and drawbridge connected the fort to the Battery Park mainland, effectively turning it into an artificial island. Landfill, however, eventually expanded Battery Park, and the fort was later incorporated into Manhattan. Thus, it’s one of New York City’s many man-made islands.