Doncaster Round Barn, Round barn in Twin Bridges, Montana.

The Doncaster Round Barn is a three-story wood-framed round barn about 1.5 miles north of Twin Bridges, Montana. Its floors step inward like a wedding cake: roughly 100 feet in diameter on the first, 75 on the second and 30 on the third, with the structure standing 48 feet to the ceiling of the top floor. The walls are three thicknesses of lumber with building paper between the layers, and the foundation rests on stone abutments quarried about five miles away.

Noah Armstrong built the barn in 1882 for breeding racehorses, including one named Spokane. That horse became the first Montana-born animal to win the Kentucky Derby in 1889.

The barn's name comes from Doncaster, the English Thoroughbred Noah Armstrong admired, and the farm was earlier called Jefferson River Ranch. From 1933 to 1997 the Bayers family raised Hereford cattle and held an annual cattle sale inside the barn; their son Byron estimated sales topped $5 million.

The first floor holds 26 box stalls arranged around a 20-foot-wide circular track. Each stall opens directly to its own outdoor paddock for the animals.

The barn's 26 stalls had dividing walls only four feet high, so horses could see one another; Armstrong believed this "neighborly companionship" relieved the monotony of indoor life. Each stall also had its own door to a separate paddock.