Bannerman's Castle is located on Pollopel Island in the Hudson River in New York, a few miles south of the Beacon-Newburgh Bridge.
The island was bought in 1900 by Francis Bannerman VI, a businessman who dealt in military surplus from the Spanish-American war. Over the next 17 years, the Scottish-style castle was built, mostly without the help of architects or engineers, to store the weapons.
The castle was abandoned in 1967. A fire on the night of August 8, 1969 gutted the castle and left it in ruins.
Bannerman's Island is being renovated and now offers guided tours!
Disclaimer: Urban exploration can be fun but dangerous. This writeup should not be taken as encouragement for unauthorized persons to visit.