Just my two cents on the pierce discussion. While spears do obviously have the same attack function as arrows, melee combat has enough nuances that I don't think that it qualifies for them to have the same type of attack. For that matter, the predominant way for a legionnaire to attack was by stabbing, but they don't get pierce attack. The basis for how easily units can defend against projectiles should mainly rest on the shield they use followed by their armour. If I were to calculate the ranged and melee armour of units, I would have a shield and armour value for them based on the type of shield and armour used. For melee attacks, the shield value would be halved and with melee attacks, the shield value would be the total amount for ranged attacks. Any attacks that get past the shield then make their way to the armour, which again detracts from the attack. If concepts like flanking are introduced on a unit to unit level, it could become even more nuanced.
Also, I think that the wall use in that way demonstrates that there should be a change in how they function. I would recommend making the turret aspect an upgrade to wall connectors. Prior to that point, ranged units would have to man them like walls for any effect.