If you think about it, Prince Caspian doesn't really do a whole lot—especially not for a title character. Other characters tend to make his decisions for him, save him from certain death, and fight his battles, too. He blows a horn, so that's something. We guess.
Still, Caspian is the protagonist because he's the character we root for. It's his war being fought, he's the one Miraz is in conflict with, and whether he wins or loses determines the fate of Narnia. Yep, sounds like a protagonist to us. (Check out his "Character Analysis" for a little more on that.)