Units in the rear may attack with their missile rating. Units in front attack with either their missile rating, or their attack rating, whichever is higher.
Thus, a noble with rating (attack=80, defense=80, missile=40) will do an attack of 40 when in the rear, and an attack of 80 when in front.
If the location of the battle is windy, all missile ratings are halved. If it is raining, the missile rating for archers and elite archers is reduced to zero.
If a fighter's missile rating is zero, it can not attack from the rear. In this case, the fighter will use its attack rating like a regular soldier.