Bannerman Castle is on Pollepel Island, which is about 60 miles north of New York City.
The abandoned castle is visible from Metro-North Railroad's Hudson train line.
To get to the castle, I took a Lyft from the Manitou train station to Donahue Memorial park in the town of Cornwall, New York.
The park sits on the west side of the Hudson River. There, I met up with guides from the Bannerman Castle Trust, the group that oversees the preservation and maintenance of the castle.
A boat waited at the park's public dock.
I crammed into the boat with two guides and a handful of volunteer gardeners who do landscaping work on Pollepel Island.
It took less than 20 minutes to get to the castle. From the dock, its beautiful, sturdy-looking walls made the building appear almost functional.
But my guide told me that I absolutely could not go inside.
From the dock, we climbed 72 stairs to reach the island.
Closer up, I understood why it's not safe to enter the castle. My guide explained that the towers of the castle require external beams for support.
The braces are made of steel. Each individual section weighs 250 pounds.
But even with this support, there's still a risk that the walls could fall.
So all visitors must stay at least 100 feet away from the castle. "Observation decks" are set up around the castle at picturesque vantage points.
A history of accidental explosions and weather damage at Bannerman Castle have left it in this decrepit state.
Francis Bannerman VI, an arms dealer who lived in Brooklyn, bought Pollepel Island in 1900. He wanted a place outside the city to store an arsenal of munitions, according to the New York Times, so he built the fortress and an accompanying harbor.
Bannerman's sons took over the business when he died in 1918. But in 1920, a room full of gun powder exploded, shattering some of the windows.
Forty-seven years later, the Bannerman family sold the island to New York State, according to the New York Times. It eventually became part of the Hudson Highlands State Park Preserve.
In 1969, a fire destroyed much of the castle, leaving it unusable.
After that, Pollepel Island was deemed unsafe and declared off-limits by the State of New York.
But in 1992, Neil Caplan, a resident of nearby Beacon, New York, formed the Bannerman Castle Trust. The group raised money to restore the island; Caplan is now its executive director.
The Trust teamed up with New York's Office of Parks, Recreation, and Historic Preservation to open the island to the public in 2004.
Today, it serves as a theater, museum, and historical site.
Although the main castle is inaccessible ...
... the Trust has stabilized the structure that served as the Bannerman family residence. That's now the island's visitor center.
The facade of the building also serves as the backdrop for the theater's stage.
Inside, visitors can find information about the island's past.
Modest exhibits explain the site's history before Bannerman built the castle there ...
... and describe the castle's construction.
Drawings and documents about the castle and the Bannerman family are framed on the building's distressed walls.
Visitors can also see preserved fixtures from the castle, like this bathtub.
Props for plays at the island's theater venue get stored inside the residence as well.
In September, the theater was showing "Dracula" ...
... so the furniture and decor in the visitor center had a spooky theme.
The center also has a gift shop that sells art and t-shirts.
Although the trust has restored parts of the castle since the '90s ...
... weather has still taken a toll on the structure in recent decades.
Much of the castle's shell collapsed one night in December 2009.
Then more walls fell just a month later, during a January storm in 2010.
Wear and tear from the elements is visible on the walls that still stand.
On the way back from the castle, the boat took a spin around the island's perimeter.
From the water, we caught glimpses of other pieces of the abandoned fortress ...
... including its deteriorated harbor.
From afar, it was especially clear that nature has already taken over many parts of this mysterious piece of history on the Hudson River.