In the beginning I was skeptical about building fortifications, but now I really like the concept. It add some dynamics into the gameplay. However, this could be improved a lot, if each class of soldier were able to build a certain class of fortification, ANYWHERE in addition to default places only.
For instance, right now what is behind a sandbag wall is predictable. You jump over, but what if the enemy could also build a trap of barbed wires behind that wall? Or what about a ladder to get up somewhere and take your enemy by surprise?
This is probably more challenge when coding the game. But already possible with mines, spawn beacons, and so on. So it is quite possible, I think. More dynamic gaming, more epic moments!