Big Shields

[Aemilius Paulus]

I don't understand why phalangites has 5 shield value: the phalanx formation could protect from arrows? How? In fact they have small shields, and when in formation they can't easily adjust their stance in order to protect from arrows.

No, that was done to make the phalanx more vulnerable to flanking attacks: shield only protects you from the left and front, not the rear and the right. In reality, however, it does not work as the soldier in the unit will simply turn around to face a flanking adversary. But now this also imbalanced the phalangites vs archers contest...

It would have been better to increase defence skill instead, which only protects a soldier from the front. And the pikes will not protect their own phalangites from arrows, only offer some meagre protection to the units behind the phalanx.

[Mikhail Mengsk]

That's why i lowered their shield value to 3.

[Lanceari]

... It would have been better to increase defence skill instead, which only protects a soldier from the front...

Unfortunately, it does not work that neatly. I quote from the RTW EDU Guide cited above:

The unit's defensive stats.
[armour] : Unit's armour value. Taken into account in all occasions (soldier attacked from any direction, melee and ranged). It measures the amount of protection a soldier's armour offers. Max value is 63 and everything higher will be considered 63.
[def_skill] : Unit's defensive skill, taken into account only in melee and only against attacks from the front or the right side. It doesn't affect defense against missiles. It represents a soldier's ability to parry(rather than block) and dodge strikes. Max value is 63 and everything higher will be considered 63.
[shield] : Unit's shield value, taken into account against both ranged and melee attacks, but only when they come from the front or the left side. Against missiles from the front it offers twice the protection it's value suggests. Measures the blocking capabilities of a unit's shield. Max value is 31 and everything higher will be considered 31.

So defense skill increases protection from front AND RIGHT ...which is not the result you were looking for.