This building was originally in another town and was moved here for exhibition. By Indian Fort it means a fort for defense against Indians, not a fort by Indians.
The description states:
THE ELM CREEK INDIAN FORT
Built in 1869
Moved from Cowles, Nebraska
30,000 items in 22 buildings on 3 city blocks, showing man’s progress over the post 120 years. 132 Miles west of Lincoln, Nebr., on U. S. 6.
— Since this museum states the starting dates for the earliest exhibits is 1830, the 120 years past mentioned above places this postcard in the 1950s.