Appart from additional entrenchment, I don't know what other benefits a unit placed on a fort tile can get. Since the additional entrenchment does not depend on the number of sides, I don't see where the importance of the number of sides is.
I have observed that the cost of some units changes suddenly. For instance, the cost of a basic army is 175 MPP's, but sometimes I can get them for 105 MPP's. Why does that happen?