Pierre Chouteau and Company, then owners of Fort William, built Fort John on this site in 1841 to compete with nearby trading posts. Constructed of adobe by Mexican craftsmen at a cost of $10,000, the fort had fifteen-foot-high walls measuring 121 by 167 feet, with living quarters, blacksmith shops, warehouses, bastions at opposite corners, and large double main doors. Indian tribes, especially the Lakota, traded tanned buffalo robes there for manufactured goods such as blankets, calicoes, guns, powder, lead, beads, looking-glasses, rings, vermillion, tobacco, and diluted alcohol. As buffalo robe trade declined through the 1840s, the fort's role shifted as westward-bound emigrants traveling to Oregon, California, and the Salt Lake Valley began stopping there, and traders did a brisk seasonal business supplying them. Poor returns in the fur trade still outweighed the summer emigrant business, and when the U.S. Army offered to buy Fort John in 1849, its owners sold it for $4,000 on June 26; the army then used the old fort mainly as a corral and warehouse until it was demolished in 1862.