Creation of specialized Soldier units who take up the same barrack space as regular Soldiers would help add an element of strategy and save some nations who would otherwise not have defenses against certain types of attack.
Being that I'm currently in an Air Defense Artilley unit in real life, I will start with that (it's short and simple):
Have an improvement that must be created, such as Patriot Missile battery (the Patriot is a defense only missile) in order to train these Soldiers.
These Soldiers, while weaker against other normal infantry, would have some air defense qualities, such as 500 of these Soldiers having roughly the same anti-air quality as 1 airplane (that is a simple example, not the proposed actual strength). These Soldiers could also have the potential to be non-deployable on the offensive battlefield since they are defensive in nature (modern real life ADA doesn't usually venture too near the front lines).
An improvement that must be created, such as a 155mm factory, to train these Soldiers.
These Soldiers are stronger than average against tanks, and possibly ships (if you had a whole lot of them), but weaker against other infantry Soldiers.
In short, artillery would give otherwise SOL military units the ability to defend themselves without necessarily developing the same exact technology in order to counter a threat, and would be interesting as it could give people being raided the defensive edge they need to overcome an enemy obstacle.