I visited this structure on October 7, 2011. It is part of the Sands Point Preserve.
Castle Gould was the name of this whole estate when it was built by Howard Gould. In 1917, Gould sold his estate to Daniel Guggenheim who kept the name Castle Gould for the stables and renamed this main house Hempstead House.
For more photos see my Sands Point Preserve Flickr page.