I love how every non-American thinks that the Civil War was apparently fought by cowboys in the middle of the desert. And they get mad at us for not learning their history.
The Civil War was vast, and America's geography is varied enough that you could have plenty of different environments to play around in. Want some open combat with horses similar to the end of AC: Brotherhood? Gettysburg, or any other huge battle! Want some stealthy sneaking around? Most Northern cities were huge, sprawling messes of factories, tenements, and other "urban development". Not too much different than London or any other modern European city at the time. And the Southern cities were also appropriately huge while retaining their own distinct charm and feel. Hell, you could design Charleston to be like Venice! Anyone who is complaining that "they wouldn't have any landmarks or history" is talking out of their ass, like they actually knew any of the "landmarks" in Florence before AC2.
The weaponry of the time was progressing quickly, but still remained "old-fashioned". You had mounted cavalry charges wielding sabers and lances fighting alongside artillery and early machine guns. Sword fighting was still popular, and though guns were coming into vogue many could not afford them for personal defense, especially not in cities.
Besides, technological anachronism is a staple of Assassin's Creed. Or did you forget that around the time Ezio was doing strafing runs in the Roman countryside?
