I picked up this game yesterday on Steam, and it is a great sim!! Very intense with simultaneous movement going on,lol No IGO/UGO type turns, this is real time style play. Think of it as Sid Meier's Gettysburg on steroids! It can handle thousands of troops on the screen at any given time, 4000 to 6000 in the first 3 hours of the battle I played yesterday! No slow down in FR either. I think its much better then the Scourge of War series covering the ACW and better then what HPS has done in their series on the ACW.
It uses the Unity Game engine, and the guys behind this sim have done extensive mods for the Total War series too. I checked the Steam forums first about this game before I bought it and everyone is having a great time with it so far, hardly any complaints. Its an early access version, about 70% complete, but very playable and a blast so far! Multiplayer is coming, but this AI in this game will give you a run for your money, it is very well done!
I mean for $9.99 you can't go wrong at this price! Here's a link for the store page on Steam-
http://store.steampowered.com/app/306660
Watch the video, its pretty amazing, but its just a small hint of what this game can do!
The designers have said you will be able to mod the game, but they don't know to what extent yet.