I visited an abandoned castle on a New York island that's only accessible by boat. It was hauntingly beautiful.

The abandoned Bannerman Castle is deteriorating on the Hudson River in New York. The island it sits on opened to visitors in 2020. National Archives and Records Administration, Joey Hadden/Business Insider When I spotted a castle in the Hudson River from my train window, I had to get a closer look. Back then, the abandoned weapon fortress wasn't open to the public. But I got a private tour. Sitting on Pollepel Island in New York's Hudson River, a stunning piece of history is slowly crumbling away. Bannerman Castle was built in the early 20th century as a fortress for weapons by Scottish arms trader Francis Bannerman, The New York Times reported. Gunpowder exploded inside the castle in 1920, and it was abandoned about 30 years later. Then, in the early 1990s, the Bannerman Castle Trust led efforts to restore the structure and make the island safe for public visitation. It's been accessible since 2004 and open for tours since 2020. I got a private tour of the island in 2019 — before tours were available to the public. Take a look at Pollepel Island's decaying castle, which is being taken over by nature.  Welcome to Bannerman Castle, an abandoned fortress in New York's Hudson River that I visited in 2019. Joey Hadden/Business Insider Today, you can reserve tours on the Bannerman Castle website. Bannerman Castle is about 60 miles north of New York City on Pollepel Island. Google Maps The abandoned castle is visible from the Metro-North Railroad's Hudson line. I took the train from Grand Central Terminal in NYC to the Manitou train station. To get to the castle, I took a cab from the Manitou train station to Donahue Memorial Park in the town of Cornwall, New York. Joey Hadden/Business Insider There, I met up with guides from the Bannerman Castle Trust, the group that oversees the castle's preservation and maintenance. I boarded a boat at Donahue Memorial Park and headed to the island. Joey Hadden/Business Insider Pollepel Island is only accessible by boat. Two guides from the Bannerman Castle Trust met me at the dock. I crammed into the boat with the guides and a handful of volunteer gardeners who do landscaping work on Pollepel Island. Twenty minutes later, I arrived at the castle. From the dock, I thought its beautiful, sturdy-looking walls made the building appear almost functional. Joey Hadden/Business Insider From the dock, we climbed 72 stairs to reach the island. But my guide told me I could not go inside — it's not safe. A view of Bannerman Castle from the north trail of Pollepel Island. Joey Hadden/Business Insider My guide explained that the castle's towers are supported by external beams braced with steel. Each section weighs 250 pounds. But even with this support, there's still a risk that the walls could fall. That's why all visitors must stay at least 100 feet away from the castle. Luckily, observation decks are set up around the castle at picturesque vantage points. Accidental explosions and weather damage have left Bannerman Castle, once a weapons storage fortress, in ruins. Joey Hadden/Business Insider 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. A room full of gunpowder exploded in 1920. Joey Hadden/Business Insider This shattered the castle's windows. Forty-seven years later, the Bannerman family sold the island to New York State. Joey Hadden/Business Insider It eventually became part of the Hudson Highlands State Park Preserve, according to the park's website. In 1969, a fire destroyed much of the castle. A view of Bannerman Castle and remnants of its harbor. Joey Hadden/Business Insider After that, Pollepel Island was deemed unsafe and declared off-limits by the State of New York. Then, in 1992, Neil Caplan, a resident of nearby Beacon, New York, formed the Bannerman Castle Trust. Joey Hadden/Business Insider The group raised money to restore the island; Caplan is 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. Joey Hadden/Business Insider Today, it serves as a theater, museum, and historical site. Although the main castle is inaccessible, the Trust stabilized another structure that served as the Bannerman family residence. That's now the island's visitor center. Joey Hadden/Business Insider 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. Joey Hadden/Business Insider Modest exhibits explain the site's history and the castle's construction. Drawings and documents are framed on the building's distressed walls. Visitors can also see preserved fixtures from the castl