What were the southern advantages at the start of the Civil War?

Despite its much smaller population—9 million to the North's 23 million—the South began the Civil War with an army almost equal in size to that of their Northern opponents. Not only that, but the South could rely on some of the finest generals in the United States—most notably Robert E. Lee, whom General Winfield Scott tried to persuade to join the Union army. Such was his reputation. Seven of the nation's eight military colleges were situated in the South, so it was no surprise that the Confederacy had so much talent on which to draw.
At the start of the Civil War, the South knew that it was going to have to fight a defensive conflict. This gave them the advantage of local knowledge. Such familiarity with the local landscape made it easier for Southern forces to harass and resist Union army incursions. The defensive nature of the conflict made the overall Southern strategy much simpler than the North's; they had to defeat a Northern invasion, thus preserving the Confederacy. The North, on the other hand, had to invade, subdue, and conquer the South, which were by no means easy tasks.