The U.S.'s largest military base during WWII, was built in 1919, and designed by Cass Gilbert (who also did the Woolworth Building).
Falling into disuse by the 1970s, the feds sold the property to New York City in 1981, and, in 1983, had the area entered into the National Register of Historic Places.
Currently, the building houses organizations such as the artist studio program Chashama, a branch of the Guggenheim Museum, and the New York City Police Department Intelligence Division.
The entire area is off-limits to nomadic or wandering sightseers, but if you'd like to see within the gates, there are tours given throughout the spring, summer, and fall, by different groups around the city.
It is worth the effort alone for a view of Building B's atrium.
They have other areas of interests, too.
Just don't walk too far off from your group.