Fort Carlos Fort San Carlos , known later as Fort Carlos , was the name given to a series of at least two simple stockade forts built during the Spanish ownership of Arkansas Post ( 1763 - 1789 ) with the purpose of protecting the trading post from attack . Both incarnations of the fort were the focus of the post , containing the commander ' s house , barrack , commissary ' s apartment , provision storage , and powder magazines . The forts were named for Charles ( Carlos ) III , who was King of Spain in the latter half of the 18th century during which Spain possessed the post . The forts were located about 10 miles from the mouth of the Arkansas River , and at a second location about 35 miles upriver , on the north bank . The remains of all the Spanish forts are now inundated by the river , due to erosion of riverbanks , changing river course , and federal construction for navigation . Fort Carlos II . When the Spanish took control of the Arkansas territory in 1763 , they decided to move their garrison from the French colonial settlement of Arkansas Post closer to the mouth of the Arkansas River . This fort was built by 1768 about 10 miles from the mouth , as it was visited that year by British Captain Philip Pittman . He described it as consisting of : " . . . a quadrangular form ; the sides of the exterior polygon are about one hundred and eight feet , and one three pounder is mounted in the flanks and faces of each bastion . The buildings within the fort are , a barrack with three rooms for the soldiers , commanding officer ‟ s house , a powder magazine , and a magazine for provision , and an apartment for the commissary , all of which are in a ruinous condition . The fort stands about two hundred yards from the water - side , and is garrisoned by a captain , a lieutenant , and thirty French soldiers , including serjeants and corporals . " In both 1777 and 1778 , the Arkansas Post was partially inundated during spring flooding . The captain , Balthazar de Villiers , wrote to the Spanish governor of Louisiana , Bernardo de Gálvez , requesting the post be moved upriver . De Villiers cited annual flooding and the long distance from the local Quapaw villages , where they were trading , as concerns . Gálvez gave permission for de Villiers to abandon the current site and move the post back to the site of the French settlement , about 35 miles upriver . There is no mention of the fort remaining in use after this abandonment . Its remains are believed to have been eroded into the river . Fort Carlos III . This was the fort built in July of 1781 at the new location of the post . The discovery of the intention by partisans to capture the post in that month prompted the fort ' s construction . Captain de Villiers wrote Governor Gálvez on July 11 regarding his intention to erect a new fort : " the King will have here a solid post capable of resisting anything which may come to attack it without cannon . " It was located on the north side of the river , approximately half a mile upriver from the post ' s habitant coast . This new location was on the Grand Prairie , a bluff above the flood plain . De Villiers described the fort ' s stockade as consisting of : " . . . red oak stakes thirteen feet high , with diameters of 10 to 15 or 16 inches , split in two and reinforced inside by similar stakes to a height of six feet and a banquette of two feet . " He also wrote that the stockade enclosed all " necessary places , including a house 45 feet long and 15 feet wide , and a storehouse , both serving to lodge my troops , and around several smaller buildings . " In addition , the fort consisted of two gates on opposite sides and two bastions at opposite angles of the fort , each mounted with 3 1 / 2 - inch brass cannons . Embrasures in the stockade , which were covered with bullet - proof sliding panels , were made for the cannons and swivel guns . Lieutenant Luis de Villars became acting commander at the post in April of 1782 after Captain de Villiers became too ill to perform his duties . Captain Jacobo Dubreuil took over as commander of the post on January 5 , 1783 , and added a bastion at one angle of the fort . De Villars remained at the fort as second - in - command , despite orders from the Spanish governor of Louisiana to leave . The fort was the focus of the Colbert Raid during the American Revolutionary War , during which it was a place of refuge for the local women and children . The side of the fort closest to the river was slowly destroyed by erosion and flooding in the years of 1787 through 1789 . After Ignacio Delinó took control of the post in 1790 , he built a new fort about a half mile back from the river , Fort San Estevan , to replace the ruined Fort Carlos . By February 1793 , the site of the former fort was entirely eroded into the river . When the United States took over the territory in 1803 by the Louisiana Purchase , it renamed Fort San Estevan as Fort Madison . It abandoned it after 1810 . Erosion continued and eventually overtook Fort San Estevan / Madison . Today , the remains of all the former Spanish forts are inundated beneath Horseshoe Lake ( Post Bend ) , the former channel of the Arkansas River now used as a navigation lake .