This guide is about 95% complete, a few things need adding but its mostly there. There currently is a perceived bug with city defence strengths which are much lower then they should be at times. It seems that something around each era is standardising the defence strengths to base values if they have not been updated. Still looking at it.
General City Combat Rules
- Combat against cities uses standard combat mechanics with some exceptions and alterations. Most notably;
- The city defence strength is a modified calculation of many factors as detailed in this guide.
- If the city has walls it severely limits a unit’s abilities to attack the cities and siege equipment typically must be used.
- A city can be put under siege to stop it healing between moves.
- Combat against a walled city counts as 2 combats. 1 to damage the wall and 1 to damage the garrison within. Both are separate combat rolls.
- A city’s garrison must be reduced to 0 HP to capture a city.
- You are not guaranteed to take a city with 0 walls and 0 garrison if you use a very weak unit. (precise details yet unknown but it is small)
- You cannot capture a city with non-military units, ranged units, siege/support units or air units (helicopters are not considered air units)
City combat Strength
- A city does not gain support bonus from adjacent friendly troops
- A city does not count as a flanking unit but any unit inside it does following normal flanking rules.
- A unit inside a city cannot be targeted directly.
- A unit inside an encampment which has been captured while the city remains in enemy hands is targeted.
- A walled city reduces all melee, cavalry and anti Cavalry damage to 15% against the wall
- Melee units (cavalry/Anti Cav and Melee) are -85% damage against walls
- A land ranged units and Air Fighters attacking city walls are firing at -17 reduced ranged strength and does 50% of normal damage.
- Naval ranged is at -50% damage but does not suffer -17 ranged strength.
- Naval melee works like land melee against cities including rams but excluding siege towers.
- When a wall is in place it reduces the garrison damage by the % of the walls HP that are left.
- The minimum damage done to walls is 1HP.
- When a city is captured, all units within it are destroyed.
- An enemy city or encampment has a zone of control and friendly units passing through a city are still affected by an enemy zone of control.
- You can select one of your combat units and then move the mouse over a visible enemy city to see how much defense it has and the likely outcome of the attack. (See picture below)
- The Base city strength is the base combat strength of the strongest land or sea unit you have built minus 10.
- As a corps, army, fleet or Armada has a stronger base combat strength these increase a cities combat strength while promotions do not.
- If a unit with a higher base combat strength than a cities base city strength occupies that city, the cities base strength is increased by the difference.
- A city displays its current combat strength on a shield next to the city name but this value does not include terrain or difficulty modifiers.Base Strength (as discussed above)
+2 per district – for each completed and non-pillaged district the city has.
+3 Ideal Terrain – If the city is on a hill
+3 Palace Guard – occurs only in your current capital city.
+5 Victors Redoubt promotion (added to the palace guard value to make it +8)
+6 Enhanced Defences – If the Bastions card has been slotted.
+3 per wall level - +3 for Ancient, +6 for Medieval and +9 for Renaissance.
+x Outside Influence (used only by City States – see city states)
+x Difficulty – an enemy major civ gains +1 defence for every level of difficulty above prince
+2 Emergency Winner – The winner of a military emergency gains +2 (ranged?)
-5 River Defence – If you attack a city over a river the city will get the standard defence modifier.
-x Damaged - (garrison damage) -1 for every 10% damage a city has taken.*
-x Unfavourable terrain – You should not see this anymore as marsh and floodplain are converted under a city but older versions and bugs may cause it.
In the below picture note the strength in the shield does not match the real city strength due to river defence, on a hill and difficulty level. The palace guard value shows on a second alternating screen due to too many modifiers.
- To capture a city you must destroy the garrison of the city
- Cities garrisons do not get military emergency benefits
- A city always has a garrison with 200 HP
- A garrison will heal 20 HP per turn unless under siege. (the Dead Sea does not affect this)
- The garrison attack is a city ranged attack which can only be used when the city has walls and these have not been reduced to 0 HP
- The garrison defends with the strength of the city
- If the city does not have walls the garrison combat counts as a standard combat
- If the city does have walls a combat is performed against the walls to reduce them and a separate combat is performed against the garrison and the damaged is based on the % damage the wall has taken.
- The unit that reduces a garrison to 0 HP counts as having killed a unit for XP assessment.
- A damaged garrison reduces city strength, a wall protects the garrison from being damaged. Using a tower starts reducing a cities strength faster than attacking the walls with a ram.
- When each of the 6 tiles that surround a city are occupied by an enemy unit or are within an enemy units ZOC (Zone of Control) the city is considered under siege and its garrison will not heal each turn. Please refer to the ZOC Guide for more information
- If the defender has units in these 6 tiles it does not negate the siege effects.
- Victors Defence Logistics promotion stops the current city he occupies from being put under siege.
- Siege units help with taking a city. There are 4 types.
- A Siege unit includes a Catapult, Bombard, Artillery and Rocket Artillery. These have a bombard value that do full damage against walls.
- An Observation unit for Siege units includes an Observation Balloon and a Drone. These increase the range of the bombard type unit.
- A siege support unit includes Battering Rams and Siege Towers. When Rams are adjacent to ancient city walls all land-based foot melee and Anti cavalry units do full damage to the walls. When Siege Towers are adjacent to a city with ancient or Medieval walls all land-based foot melee and Anti cavalry units do full damage to the Garrison and ignore the walls. If both siege support units are adjacent to the same city with ancient walls, then all land-based foot melee and Anti cavalry units do full damage to both walls and Garrison.
- A bomber unit includes a Bomber and a Jet Bomber which have bombard values that do full damage against walls but do not gain benefit from observation units beyond their visibility.
- A city with a wall has a ranged attack which acts just like any other ranged attack but is not modified beyond civic cards specific to walls attacks.
- A city attacks with the highest base range strength of any land or sea ranged unit the civ has built.
- If a civ has not built any ranged units, a city fires with a strength of 3.
- An encampment cannot fire on any enemy encampment or city but a unit within an encampment can.
- A city will gain +5 to ranged strength is the Bastions card is slotted.
- A city gains 2 shots with Victors Embrasure promotion.
- A city ranged attack will target with the same rules as ranged, the weakest modified strength unit within range.
- 1 production per 10 HP repaired
- Walls must be repaired before you can upgrade them
- If both walls are damaged only the most expensive (if you can call it that) counts.
- 3 complete turns without being attacked before you can repair them.
- When you capture a city after discovering Urban Defences the captured city starts with half HP walls and garrison.
- Encampment Garrisons only have 100 HP but their wall HP match their city center.
- Their combat strength is the same as a city but does not include any garrison bonus.
- When an encampment is occupied all its buildings are automatically pillaged.
- When a captured encampment is vacated it starts healing at 20 HP per turn and will count again as enemy occupied for ZOC and ownership.
- City states act in most ways like normal city walls. There are 3 exceptions
- City State walls start with an additional 5 strength
- City states to not have a difficulty modifier
- City states gain +1 strength per envoy
- City states typically build 1-2 districts but could have more if they had been captured, built up then freed.
Akkad City State
Tips and tricks
- Melee and anti-cavalry units' attacks do full damage to city's walls without a RAM.
- This benefit if only for land-based foot units of the class Anti Cavalry or Melee.
- Normally these units only do 15% of normal damage to city walls while being Suzerain of Akkad allows full damage to both city and encampment walls
- Akkad also affects modern walls gained at the Steel Tech but remember they have 400HP.
- Regardless of the way you attack a city you must be aware of which unit the city will shoot at. This allows you to minimize losses and misdirect fire. For example, attacking with a catapult and a warrior, the city will attack the warrior but attack with catapult and swordsmen and the city will attack the catapult so it may be best to have a non-upgraded warrior to misdirect fire from the catapult.
- Placing a city under siege can speed up the taking of the city significantly if you do not have enough strength to take the city fast. Be aware of what units you have that have a ZOC that can help this and if the city is adjacent to water we aware that embarked land units only have a ZOC in the water tile they occupy.
- Using an observation unit with a siege unit allows the siege unit to be out of range of the city ranged attack while it attacks.
- When attacking a city with walls it is normally best to attack with siege units first, then ranged and finally with melee units. This is because the walls severely limit the damage the garrison can take and so reducing the walls first with units that do not take damage means the typically heavier hitting melee units can cause a larger damage against the garrison. The first attack will pretty much only damage the wall and even attacking with a scout will reduce that wall % to 99% so a melee unit at least starts damaging the garrison.
- Be very aware of cities spawning additional units. You are often better off bleeding a civ’s units dry before attacking cities as they can buy units which, if the city is under siege will get placed outside of your units and can cause significant interference if you cannot take the city fast.
- When attacking a civ early in the game that has not yet got chariots, always assume they will build a chariot in a city when you attack it, otherwise you may find yourself too deep in enemy territory with a disappearing army.
- Loyalty does not affect combat (rightly or wrongly) so do not think that a disloyal city is going to be easier to take.
- Attack an encampment with a siege tower as they have half the garrison of the city.
This material was first investigated in thread https://forums.civfanatics.com/threads/city-strength.619428/
It has since been tested again to validate results, but these validations by and large have not been posted anywhere. If there is a conflict in understand please feel free to contact me, we all make mistakes.
A guide to how city defence and combat works