Naval units, and land units with the Heavy Artillery ability may conduct Bombardment by selecting the Long Range Fire option from the Action menu.
Bombardment has the advantage of potentially damaging every unit in a particular square and the disadvantage that land units can never be reduced below 50% damage by bombardment.
If bombardment is attempted against a square containing an enemy naval or artillery unit, the bombardment is cancelled an an Artillery Duel is initiated, with the attacking and defending artillery (or naval) units fighting attack-vs-attack strength. Armor values are ignored for Artillery Duels.