Artillery shells deal splash damage, so if two or more buildings are adjacent to each other, an Artillery shell can deal its full damage to all of them if directed at the center of the building cluster.
Check out our Artillery Hit-to-Kill Calculator to find out how many Artillery shots it takes to destroy any building.
Do not place buildings adjacent to each other, not even adjacent at a corner, as they can all be damaged or destroyed at the same time.
Upgrade defenses that are common Artillery targets to increase their health. If a defense has a lot of health, an attacker will have to use more energy to destroy it. Building Health Statues can also be used for this reason.
There are no visual differences between upgrades. However, damage increases by 11% with each level.