The barricade is counter-intuitive.
The "stopping action" of the barricade only occures if the enemy unit actually enters the barricade tile itself. That's it. This results is the enemy now sitting in a heavily fortified position.
Granted, you have time to react. But this hardly helps you establish a sane defense. No defense would better the enemy's position.
It also goes against logic in that you first build the fortress which provides an improved defense. Second, you "upgrade" this fortress which is supposed to provide two benefits -- 1) an even stronger defensive position, and 2) the "stopping power." However, to get the "stopping power" you need to abandon the position to your enemy.
Judging from most of the posts in this thread, people are hypothisizing about the way barricades would intuitively work... as in my earlier post and Bambul's. (Barricades would stop units that travel withing one hex of the barricade).