Fort Carlos III was a Spanish fort located at the Écores Rouges, Luisiana, within the present-day Arkansas Post National Memorial. Named for King Carlos III, it was located on the left bank of the Arkansas, about 29 miles from the mouth. This wooden palisade fort comprised a stockade, bastions, embrasures, storehouses, barracks, officers' quarters, a powder magazine, cannons, swivel guns, and ordnance stores. The fort served as the headquarters of Spain's district of Arkansas from 1781 until 1791 when it was destroyed by erosion and replaced by Fort San Esteban.