If you want to weaken artillery, make the ammo cost money, and they can only carry a few rounds at a time. That way you can't just pull up a big army and then hide some artillery behind them and pound away at their base. I don't like games where you can do something like that. If they can't overpower your army, they can't kill the artillery, and if they sit in their base and do nothing, they suffer a slow death. It makes base defenses useless. All they'd do is slow down the attack. If you want a form of bombardment like that, you either need to make sure it's really easy to kill, or it's rate of doing damage is so slow that the person being attacked can compensate for it in some way.
If they had to return to base to reload their ammo, it would significantly slow down their rate of damage. Or, if you decreased their attack range to some medium value, where they outrange other weapons but not by much, then you can't hide them behind an army. If they needed to be positioned at the front of the army to be within range, it would make them easier to kill. Especially if they were slow turning and moving since they wouldn't be able to run away. Instead, to protect them, you'd have to rush your units up, possibly within range of their defenses. And even then, you can't stop people from targetting your artillery, and then pulling back.
Also, I like the idea of some sort of delay for the first attack. Like how siege tanks in StarCraft need to deploy before they fire. That gives tanks in a defensive position an advantage, so if you're gonna attack with artillery against artillery, then the attacker either needs to seriously outnumber them, and expect the first few units to be slaughtered, or they need to provide support with other units, possibly strong defensively against artillery.
So yeah, basically artillery shouldn't be some sort of super weapon. Just a useful support.
And yeah, I think Lev is right. Artillery doesn't seem very Outpost style of a unit.