 An important <unk> of the British and American frontier was a system of forts built by the military . The forts provided safe shelter for soldiers and explorers on the frontier and a base of operations for expeditions , both military and commercial . The first forts in the area had been French , particularly Fort <unk> , built during the 18th century and later abandoned because of the French and Indian War with the British . British Fort Charlotte at Grand Portage became essential to the fur trade protecting and supplying British traders as well as the area natives . This British fort operated in the area ( illegally ) until 1803 , even after the area 's becoming recognized as part of the United States . Other French and British fortifications , such as Fort St. Charles , had existed in the region but had been abandoned much earlier . 
