someone said that if you attack behind a large (+2) shield unit, you get +2 attack bonus (your enemy don't lost shield bonus but you get att. bonus). No mention of bonus in side attack.

In another test, the archers shoot effective to the back of their targets.

So I think shield add bonus to armor and defense, in front and sides. Very good And this +2 bonus don't give the armor piercing enemy unit a + 1 attack as body armor