Fort Armstrong was built in 1816-1817. Its riverside was protected by limestone bluffs and its other sides were formed in part by the rear walls of barracks and storehouses. Blockhouses stood at three corners. The fort was garrisoned by United States troops until May 4, 1836. It served as headquarters for the Sauk and Fox Indian Agent from 1836 to 1838 and as a military depot from 1840 to 1845. It was destroyed by fire in 1855.