New York City:

New York City is home to Fort Hamilton, the U.S. military's only active duty installation within the city. Established in 1825 in Brooklyn on the site of a small battery utilized during the American Revolution, it is one of America's longest serving military forts. Today Fort Hamilton serves as the headquarters of the North Atlantic Division of the United States Army Corps of Engineers as well as for the New York City Recruiting Battalion. It also houses the 1179th Transportation Brigade, the 722nd Aeromedical Staging Squadron, and a military entrance processing station. Other formerly active military reservations still utilized for National Guard and military training or reserve operations in the city include Fort Wadsworth in Staten Island and Fort Totten in Queens.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". When was Fort Hamilton built?
1825