In the time of a great civil war, a young man is positioned in a tower alone where he must fight for survival with very few supplies, and a sniper rifle.

At the risk of being a buzzkill ... isn't this the plot to the movie-within-a-movie at the centre of Inglourious Basterds?

Overall I think the logline is good. I agree that it would to establish what civil war you are referring to. It would help a reader establish if it is a period piece or modern day.

Good points Torie & Timmy. much appreciated.
Those ideas would work well in the logline to establish the atmosphere of the situation.
But for me, it was more of the IDEA, that a young boy fresh out of school volunteered for the army and got positoned alone on a tower/rooftop during a war with a few supplies. So I didn't really think it would matter what/when the battle was. In the logline atleast.
Both work well. I'll probably change/add some words to it, to make more clearer. :)