Now, building a fort outside of town to defend it is a legitimate tactic in the game.

There's one caveat, though: Make certain the zone-of-control of the troops in the fort extends beyond the far side of the town you're trying to protect.

I learned this the hard way in my Dutch campaign. Late in the game, Sweden finally took out Hannover and I purchased it from them. Since I was still at war with Prussia I moved troops into the province immediately, and I built a fort a little east of the capital. I wanted to be able to protect both the capital and the two towns near it, and the ZOC of the army was touching all three. Unfortunately, when Prussia decided to raid the closest town, the fort did not protect it. They moved in without opposition. Once they moved OUT towards the capital I got to intercept them, but it was a little late at that point. If the ZOC extends out on the opposite side of the town you're protecting, you should get the chance to intercept an attacking army, which will force them to assault and take your fort.