Richmond, Virginia:

There are also parks on two major islands in the river: Belle Isle and Brown's Island. Belle Isle, at various former times a Powhatan fishing village, colonial-era horse race track, and Civil War prison camp, is the larger of the two, and contains many bike trails as well as a small cliff that is used for rock climbing instruction. One can walk the island and still see many of the remains of the Civil War prison camp, such as an arms storage room and a gun emplacement that was used to quell prisoner riots. Brown's Island is a smaller island and a popular venue of a large number of free outdoor concerts and festivals in the spring and summer, such as the weekly Friday Cheers concert series or the James River Beer and Seafood Festival.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". During what conflict was Belle Isle used as a prison?
Civil War