From Wikipedia:
Fort Koshkonong (Fort Cosconong) was located at a site which today is in the city of Fort Atkinson, Wisconsin, about three blocks east of Main St. along Milwaukee Ave. General Henry Atkinson was the commander of the fort during the Black Hawk War. It was used as a station for local milita units and the US regulars in the region to scout the British Band. Black Hawk was in the same general area, but evaded capture and started to flee towards the Wisconsin River. The original fort was abandoned following the conflict and was dismantled by local settlers for the wood. The fort's location is along the Rock River and is marked with a monument. There is also a replica of the fort... It's over all size is somewhat larger than the actual fort, but gives a good representation of how it looked during the Black Hawk War.
Fort Koshkonong shares its name with Lake Koshkonong, which is several miles downriver. That lake is very shallow, and at the time the fort was active, before it was dammed, was actually more of a marsh than a lake.
From the Wisconsin Historical Society web site:
On this day in 1832, General Henry Atkinson finished erecting at the junction of the Rock and Bark Rivers a temporary stockade known as Fort Koshkonong. He was hot on the trail of Black Hawk, who had retreated up the Rock River and was then about 60 miles away, just east of Fort Winnebago at Portage.
All but 30 of the troops normally stationed at Fort Winnebago were in the field with Atkinson when the non-combatants there received word that the Sauk warriors intended to attack it. A 16-year-old clerk named Satterlee Clark was chosen to sneak downriver to Fort Koshkonong and get help:
"At nine o'clock at night I left the fort," he later recalled, "with many a God speed you, armed with a small Ruggles rifle, my dispatches, a tomahawk and bowie knife. I crossed the Fox river at a shallow point just above where the public stables used to stand, and keeping the Indian trail that led from here to White Crow's village on Lake Kosh-ko-nong on my right, I traveled rapidly all night, walking uphill and running downhill and on a level. I struck the trail several times during the night but left it immediately, as I feared some Indians might be encamped upon it whose dogs would discover me before I would discover them.
"I arrived safely at the fort [Koshkonong] at half past 11 o'clock in the forenoon and delivered my dispatches to Gen. Atkinson, who sent 3,000 men at once to relieve Fort Winnebago. I may add that Fort Atkinson was constructed of log pickets with loop-holes for musketry, with block-houses on the south-east and north-west corners, with about an acre of ground within the enclosure. I slept till 4 o'clock in the afternoon and then started on my return, following the trail of the mounted militia for twelve miles, when I passed them and reached the head of a stream that used to be called Rowan's Creek, about twelve miles from the fort, shortly after daylight; and fearing to go further till night, I crawled into some brush and went to sleep. As soon as it was quite dark, I left my hiding place and returned to the fort as near as possible by the route I left it, arriving between ten and eleven o'clock p.m." He had covered more than 100 miles on foot in 48 hours. Atkinson's troops arrived the following afternoon and the rumored attack was averted. [Wisconsin Historical Collections 8: 313-314]
Meanwhile, back at Fort Koshkonong, civilians rushed into the stockade for protection. Celeste Foster was the first child to arrive, and in 1874 she recalled these details:
"The stockade was constructed of burr oak logs cut in half, the ends of which were set in the ground so as to stand about eight feet hish. It was located a little east and north of where the residence of E.P. May now [1874] stands [about a mile east of downtown]. At one time there were 4,500 United States troops under the command of Gen. Atkinson around the Fort."
Local tradition says there was never a building at the fort, but only a stockade fence topped by blockhouses at the corners. It was never needed as a refuge, since by the time it was built Black Hawk's warriors had already passed through the region and moved on toward Madison. It was occupied for only two months, as a supply post where food and other necessities could be assembled and distributed to the soldiers in the field.
The first settlers arrived four years later and reportedly used its timber to build log homes, though some of the burr oak posts and other evidence of Atkinson's occupation were visible until at least 1840. A hand-drawn map of Fort Koshkonong dating from 1835 is housed in the Hoard Historical Museum. The D.A.R. marked the fort's location with a monument in 1907, and a reconstruction of the fort was built in 1966 at Rock River Park in Fort Atkinson.