I figured I'd start an issue on bombardment just to gather information in one place before I get too deep into implementing things.
Some pickings from the official guide.
Overview
When you give a unit the order to Bombard, your cursor changes to cross-hairs. Use the cross-hairs to select the target of the bombardment. (If the cursor becomes anything other than cross-hairs, you’ve moved it outside the effective range of the bombardment.) Only certain units have this ability; check the Civilopedia entry for a unit if you suspect it might be capable of bombardment.
Bombard ability: Warships, bombers, fighters, and artillery units have the ability to bombard a target that’s within their range. Bombardment counts as a unit’s attack, and might damage defensive fortifications, harm units, or otherwise damage a city in the target square.
Bombardment
Artillery units (Catapults, Cannons, and all Artillery units) and warships (Frigate, Man-o-War, Ironclad, Destroyer, Battleship, Aegis Cruiser) have the ability to bombard a target that’s within their range.
Bombardment is an attack that does not involve moving into the same square as the defender. It’s a “stand-off” or “ranged” attack. The attacker takes no risk of damage.
Bombardment affects everything in the target square, not just enemy units. The projectiles you launch might damage defensive fortifications like Fortresses and city Walls, harm military units, destroy a portion of a city’s population, or demolish city improvements.
Note that the bombing attacks of fighters and bombers work in much the same way as this type of bombardment.
Nulear Attacks
You launch a nuclear attack in the same way you target an air unit bombing mission.
All units in the target square and adjacent squares have only a 50% chance of surviving, regardless of their cultural allegiance (in other words, both theirs and yours). In addition, a bombed city loses half its population. The defense against most nuclear attacks is the Small Wonder SDI Defense.
City Governor
[City governors can be set to] "Artillery – strictly offensive bombardment units, like Catapults".
Terrain Damage
Terrain bombardment can cause craters that can be cleaned by Workers. Pressing [Shift]-[D] will automate a unit to clear damage.
Orders
Air Mission: Bombing mission
Bombing mission ([B]): Drop bombs on the selected terrain square or enemy city. Air bombardment affects city improvements and city populations.
Air Mission: Precision bombing
Precision bombing: Drop bombs that target improvements. This mission is only available if your civilization has researched the Smart Weapons advance for Stealth Bombers and Stealth Fighters.
Bombard ([B])
Use this to order a unit capable of bombardment to use that ability to damage any suitable target within range.
Automated Bombard, Bombing, and Precision Bombing
You can order units to bombard, bomb or precision bomb a single target repeatedly without having to issue a new order every turn.
The keyboard shortcuts for these actions are always available. You must turn on Advanced Unit Action buttons in the Preferences screen if you want the following buttons for these commands to be displayed on screen when you select a unit.
Note: Automated bombardment, bombing, and precision bombing continue until you stop the attack.
Units
Naval Units
Many naval units can conduct bombardment — that is, they can bombard units or cities on land squares. This type of bombardment works in much the same way as the Bombard ability of ground units.
Nuclear Submarines can carry Tactical Nukes. No other subs can carry any other kind of missile.
Air Units
Bombing Mission: Bombard on the selected terrain square or enemy city. Air bombardment affects units, city improvements, and city populations.
Precision Bombing: Once your civilization has researched the Smart Weapons advance, Stealth Fighters and Stealth Bombers can execute this mission. Useful only against cities, precision bombing targets improvements. If all improvements in a city are destroyed, the mission then targets population.
Certain air units can carry out these missions, but only if a suitable target is within its operational range. The range is outlined on the map in the same way as the range for bombardment, and the same cross-hairs help you to find appropriate targets.
Missiles
If a city is the target of a normal cruise missile attack, the city suffers a bombard attack. A nuclear missile attack destroys half the population, regardless of nationality. Military units have a 50% chance of surviving a nuclear attack. In addition to the loss of units, cities, and improvements, all land terrain squares adjacent to the impact square become polluted. As you might expect, all missile units are one-shot attackers. They’re always destroyed as part of the process of attacking.
Note that missiles are considered air units and function identically to planes with regard to movement.
Nuclear units not only destroy the army or city they target, but all units stacked with the target and those in adjacent squares as well. The detonation also pollutes and devastates a number of map squares around the impact square. Your rivals might not spend the time or manpower to clean it up, but if you ever intend to use those squares, you should consider it.
Other things
Diplomacy
Breaking alliances or treaties can blacken your reputation in the international community. Savagely razing the city of an enemy or using a right of passage agreement to set up Cannons to bombard your opponent’s cities are acts likely to be deplored throughout the known world.
Artillery capture
If there’s no defender nearby, any military unit (one with an attack factor) can take control of a unit that’s incapable of defending itself.
You can capture artillery units (Catapult, Cannons, and such), but only if you already have the advance that would allow you to build the unit. That is, if your civilization doesn’t yet understand how a unit works, you can’t capture it and use it.
Impassable terrain
Certain terrain is impassable to certain types of units.This is terrain that those units cannot traverse, usually due to physical limitations. For example, wheeled units like Catapults and Cannons cannot travel across mountain squares unless someone has built a road through the range.
I figured I'd start an issue on bombardment just to gather information in one place before I get too deep into implementing things.
Some pickings from the official guide.
Overview
When you give a unit the order to Bombard, your cursor changes to cross-hairs. Use the cross-hairs to select the target of the bombardment. (If the cursor becomes anything other than cross-hairs, you’ve moved it outside the effective range of the bombardment.) Only certain units have this ability; check the Civilopedia entry for a unit if you suspect it might be capable of bombardment.
Bombard ability: Warships, bombers, fighters, and artillery units have the ability to bombard a target that’s within their range. Bombardment counts as a unit’s attack, and might damage defensive fortifications, harm units, or otherwise damage a city in the target square.
Bombardment
Artillery units (Catapults, Cannons, and all Artillery units) and warships (Frigate, Man-o-War, Ironclad, Destroyer, Battleship, Aegis Cruiser) have the ability to bombard a target that’s within their range.
Bombardment is an attack that does not involve moving into the same square as the defender. It’s a “stand-off” or “ranged” attack. The attacker takes no risk of damage.
Bombardment affects everything in the target square, not just enemy units. The projectiles you launch might damage defensive fortifications like Fortresses and city Walls, harm military units, destroy a portion of a city’s population, or demolish city improvements.
Note that the bombing attacks of fighters and bombers work in much the same way as this type of bombardment.
Nulear Attacks
You launch a nuclear attack in the same way you target an air unit bombing mission.
All units in the target square and adjacent squares have only a 50% chance of surviving, regardless of their cultural allegiance (in other words, both theirs and yours). In addition, a bombed city loses half its population. The defense against most nuclear attacks is the Small Wonder SDI Defense.
City Governor
[City governors can be set to] "Artillery – strictly offensive bombardment units, like Catapults".
Terrain Damage
Terrain bombardment can cause craters that can be cleaned by Workers. Pressing [Shift]-[D] will automate a unit to clear damage.
Orders
Air Mission: Bombing mission
Bombing mission ([B]): Drop bombs on the selected terrain square or enemy city. Air bombardment affects city improvements and city populations.
Air Mission: Precision bombing
Precision bombing: Drop bombs that target improvements. This mission is only available if your civilization has researched the Smart Weapons advance for Stealth Bombers and Stealth Fighters.
Bombard ([B])
Use this to order a unit capable of bombardment to use that ability to damage any suitable target within range.
Automated Bombard, Bombing, and Precision Bombing
You can order units to bombard, bomb or precision bomb a single target repeatedly without having to issue a new order every turn.
The keyboard shortcuts for these actions are always available. You must turn on Advanced Unit Action buttons in the Preferences screen if you want the following buttons for these commands to be displayed on screen when you select a unit.
Note: Automated bombardment, bombing, and precision bombing continue until you stop the attack.
Units
Naval Units
Many naval units can conduct bombardment — that is, they can bombard units or cities on land squares. This type of bombardment works in much the same way as the Bombard ability of ground units.
Nuclear Submarines can carry Tactical Nukes. No other subs can carry any other kind of missile.
Air Units
Bombing Mission: Bombard on the selected terrain square or enemy city. Air bombardment affects units, city improvements, and city populations.
Precision Bombing: Once your civilization has researched the Smart Weapons advance, Stealth Fighters and Stealth Bombers can execute this mission. Useful only against cities, precision bombing targets improvements. If all improvements in a city are destroyed, the mission then targets population.
Certain air units can carry out these missions, but only if a suitable target is within its operational range. The range is outlined on the map in the same way as the range for bombardment, and the same cross-hairs help you to find appropriate targets.
Missiles
If a city is the target of a normal cruise missile attack, the city suffers a bombard attack. A nuclear missile attack destroys half the population, regardless of nationality. Military units have a 50% chance of surviving a nuclear attack. In addition to the loss of units, cities, and improvements, all land terrain squares adjacent to the impact square become polluted. As you might expect, all missile units are one-shot attackers. They’re always destroyed as part of the process of attacking.
Note that missiles are considered air units and function identically to planes with regard to movement.
Nuclear units not only destroy the army or city they target, but all units stacked with the target and those in adjacent squares as well. The detonation also pollutes and devastates a number of map squares around the impact square. Your rivals might not spend the time or manpower to clean it up, but if you ever intend to use those squares, you should consider it.
Other things
Diplomacy
Breaking alliances or treaties can blacken your reputation in the international community. Savagely razing the city of an enemy or using a right of passage agreement to set up Cannons to bombard your opponent’s cities are acts likely to be deplored throughout the known world.
Artillery capture
If there’s no defender nearby, any military unit (one with an attack factor) can take control of a unit that’s incapable of defending itself.
You can capture artillery units (Catapult, Cannons, and such), but only if you already have the advance that would allow you to build the unit. That is, if your civilization doesn’t yet understand how a unit works, you can’t capture it and use it.
Impassable terrain
Certain terrain is impassable to certain types of units.This is terrain that those units cannot traverse, usually due to physical limitations. For example, wheeled units like Catapults and Cannons cannot travel across mountain squares unless someone has built a road through the range.