W. C. Child Ranch, near Helena, Montana, ca. 1890,* from an Historic American Building Survey (HABS), via Library of Congress Prints and Photographs Division.
Mr. Child became rich from prospecting in Montana. He built this octagonal house on his 3,000-acre ranch in the late 1880s.
However, he used it not as a home, but as a party space. (The whole second floor was a ballroom.) He and his friends — sometimes over 100 — would take the Northern Pacific train from nearby Helena for banquets and dances lasting late into the night.
By 1893, Child was broke and had to assign the ranch to another man. He was found dead in the house a month later.
Child called the ranch “White Face Farm” for the Hereford cattle he raised there, and he built Montana’s largest barn to protect them during the winters. There are more details here.
The house and barn still exist as a special events center called Kleffner Ranch. *Both HABS pictures here were photocopies of original photographs; the originals are in the collection of the Historical Society of Montana.
enclos*ure 
Is there something about octagonal buildings? I seem to remember an octagonal house built in Natchez (or another similar southern town on/near the Mississippi) that was started by a man who became wealthy, lost his fortune and never finished the house. Memory may be faulty, though… Still, we have two octagonal barns near us in rural Quebec, both built 100+ years ago and still going strong.
There’s a wonderful 1801 octagonal house in the Foggy Bottom neighborhood of Washington, D.C. It’s owned by The American Institute of Architects and open to the public a few days a week (http://www.aia.org/conferences/the-octagon).
I’ll look for it the next time I’m in Washington. Thanks for the pointer.