The following units can be upgraded to Artillery:
Artillery units are very effective at attacking enemy cities. They can bombard enemy city fortifications and also cause collateral damage to other enemy units sharing the target's space.
Artillery is a general term covering several varieties of large-caliber weapons including mortars and field guns. Mortars fire an explosive projectile in a high arc, and are particularly effective against small, concealed targets. Field guns, long-range weapons that fire their projectiles in a flat arc, are used primarily for their ability to penetrate hard targets. Powerful enough to batter opposing forces even behind the strongest fortifications, artillery is also able to move quickly from one firing position to another. This is an important capability when fighting a moving battle or when avoiding enemy fire.