Combat

Combat in Civilization 4 compares the attacker's and the defender's Strength after modifiers (terrain, fortification, city and culture defense, promotions, bonuses against unit types); equal Strength means even odds and double Strength 2 to 1, and the loser is destroyed unless it withdraws. Any number of units can share a tile, the best defender always fights, and siege units' collateral damage is the answer to the stack.

Combat in Civilization 4 compares the attacking and defending units' Strength after every modifier (terrain, fortification, city defenses, promotions and bonuses against unit types) and fights until one of them is destroyed or withdraws: if the two modified strengths are equal each side has an even chance, and if one is double the other its odds are 2 to 1, the Civilization 4 manual explains (p. 40). The winner may come out wounded and weaker until it heals. Any number of units can stand on one tile, and when a stack is attacked the defender with the best odds fights for it, which is why siege units and their collateral damage matter so much.

3D map of hills and forest. Mounted knights in red and white, foot soldiers and a catapult gather by a snow-capped mountain; a city label reads Athens with +52% beside it, and another label at the top right reads Tlatelolco, Catapult. — Gameplay on 2K's Civilization IV page: an army of knights, musketmen and catapults beside a mountain outside Athens, whose label shows a +52% defense bonus, facing the border of Tlatelolco. Firaxis Games and 2K

Quick facts

Odds	Equal modified Strength: even; double Strength: 2 to 1
Attacks per turn	1 (Blitz allows more); a defender can be attacked any number of times
Fortify	+5% defense per turn, up to +25%
Fort	+25% defense
Forest or jungle	+50% defense (melee, archery and recon units)
Hills	+25% defense
From a boat	-50% Strength unless Amphibious
City defense	Walls and Castles or culture, whichever is higher; Musketmen and later units get only culture
Stacks	The best defender fights; the tile falls only when every defender is dead
Healing	Fastest in your own territory, slowest in enemy territory

Source: Civilization 4 manual, pp. 38-49, 108-110 and 124-125.

How a battle is decided

A battle starts when a unit tries to enter a tile held by a hostile unit. The Civilization 4 manual (pp. 39-40) says the game compares the two combat strengths, modified for terrain and special abilities, and generates a result: a losing attacker is destroyed and the defender stays; a losing defender is destroyed and the attacker moves in if the tile is now empty. Units of civilizations at war are hostile; barbarians and animals are hostile to everyone. Moving onto a neutral unit's tile asks whether to attack, which declares war.

Damage carries over. A unit that survives a fight keeps its injuries, and its Strength stays lowered until it heals, so a wounded defender is easier to beat with the next attack, the manual (pp. 41, 43) notes. Most units can attack only once per turn even with moves left, but a defender can be attacked any number of times in a turn as long as it survives.

Civilization 4 shows the odds before an attack. Johnson later wrote, in a Game Developer column of Mar 16, 2010, that the help mode showing exact combat odds "drastically increased player satisfaction" with combat.

What changes a unit's Strength

The modifiers the manual (pp. 41-42) lists are injury, fortification, a Fort, defensive terrain, city defenses, bonuses against unit types and promotions. Each is a percentage of the unit's Strength: a Spearman's +100% against mounted units doubles its Strength from 4 to 8, and an Archer's +50% city defense makes it a 4.5 in a city, the manual's own examples (pp. 33-34).

Terrain helps the defender. Forest and jungle give +50% and hills +25%, per the manual's terrain list (pp. 108-110), though mounted and siege units get nothing from them (p. 41). An attacker crossing a river is weakened, the manual says without a number (p. 31), and a land unit attacking from a boat fights at -50% unless it has the Amphibious promotion, which also removes the river penalty (pp. 124-125, 136).

Fortifying adds +5% defense for every turn a unit stays put, to a maximum of +25%, and is lost when the unit moves or attacks; a Fort adds +25%, the manual says (p. 41). Mounted units cannot fortify. In the expansions Forts can be built outside your borders and give attack, defense and healing bonuses, per the Beyond the Sword manual (p. 46).

City defense and culture

A city gives its defenders either its physical defenses (Walls and Castles) or its cultural defense bonus, whichever is higher, never both, the Civilization 4 manual (p. 49) explains. Musketmen and every later unit get nothing from Walls and Castles, only from culture. The bonus shows beside the city's name on the map and on the city screen, as on Athens's +52% in the first image on this page. A city in resistance after a capture loses its cultural defense.

Siege units and some ships and aircraft bombard a city's defenses down before an assault: a Catapult lowers them by 15% a turn, a Trebuchet or Artillery by 25%, a Cannon or Battleship by 20%, per the Warlords manual's charts (pp. 119-128). Bombardment always succeeds, never harms the units inside, and stops at zero (base manual, p. 42). The Chichen Itza wonder gives +25% defense in all its owner's cities; each City Raider promotion adds +20% to +30% to attacks on cities, and each City Garrison promotion the same to their defense.

First strikes

A first strike is a chance to damage the enemy before it can do any damage back, the Civilization 4 manual (p. 33) says of the Archer, and it is canceled when the other unit has first strikes too. Archers, Longbowmen, Crossbowmen, Machine Guns and Modern Armor start with one, and the Drill promotions add more. Some units are immune to first strikes: Horse Archers and Knights, and any unit with Flanking II, per the Warlords manual's charts and the base manual (p. 142).

Stacks and collateral damage

When a unit attacks a tile with several defenders, the game picks the defender with the best odds against that attacker, and the attacker cannot move in until every defender is dead, the Civilization 4 manual (pp. 41-42) explains. Naval and air units in an attacked city take no part, and are destroyed if it falls.

Soren Johnson's afterword to the manual (pp. 182-183) names the problem this creates, which the fan community called the "Stack-of-Doom": if every unit has a counter, the best play is one giant stack, since the best defender is always there. The team's answer was a milder form of Civilization 1's collateral damage. Catapults, Cannons and Artillery are "the stack-killers", a counter "for a style of play" rather than for a unit type, which creates tension between keeping units together for defense and spreading them out to avoid collateral damage.

3D close-up of archers drawing bows and soldiers with quivers beside white banners with a red cross, a small walled town labeled Nottingham, Worker, and a boat on a blue inlet. — Gameplay on 2K's Warlords page: archers with red-cross banners stand around the small city of Nottingham by a lake, with a boat on the water. Firaxis Games and 2K

A siege unit's barrage wounds several units in the defending stack when it attacks, and the Barrage promotions raise that collateral damage by 20%, 30% and 50%, per the manual (pp. 42, 137). Bombers and Battleships deal collateral damage too. On defense, the Warlords-era patches made Drill II, III and IV each give 20% protection from collateral damage, per the Warlords manual (p. 103).

Beyond the Sword limited siege units: each now has a Maximum Damage limit, deals damage up to it and withdraws, and cannot attack a unit whose health is already below that limit, per the Beyond the Sword manual (p. 24). Some mounted and helicopter units gained a Flank Attack: attacking a stack that holds siege units, they damage those siege units as long as they survive the fight, even when they withdraw.

Withdrawal

Some units can escape a losing attack. Mounted units may withdraw when they are about to be destroyed attacking a non-mounted unit, never when defending, the Civilization 4 manual (p. 43) says. The Warlords manual's charts give the chances: Cavalry 30%, Catapult and Gunship 25%, Horse Archer 20%, Chariot 10% since the patches, Submarine 50%. The Flanking promotions add 10% and 20%, and a Great General's Tactics promotion 30%.

Healing and capture

A unit that does not move heals part of its damage each turn: fastest inside your own borders, slower in neutral land and slowest in enemy territory, per the Civilization 4 manual (p. 43). Medic I adds 10% a turn for every unit in its tile and Medic II for adjacent tiles as well, Combat IV and V speed healing in neutral and enemy lands, and March lets a unit heal while moving; see Promotions.

Entering a tile with an enemy Worker captures it, and a captured Settler becomes a Worker; units earn no experience from it. Invisible units such as Spies cannot be harmed in combat, the manual (p. 42) adds.

Why it works this way

Civilization 4 merged attack and defense into one Strength value "to create design space" for situational bonuses, Johnson writes in the afterword (p. 182), so that combined arms beat any single unit type; see Units for the counters. GameSpot's review of October 2005 noted the new system avoids the old "spearman beats tank" result, and that artillery "has been tweaked to be much more useful".

Stacking is what the next game changed: PC Gamer wrote on Jan 28, 2025 that some long-time players "still complain about the decision to end unit stacking in Civ 5". Jon Shafer, lead designer of Civilization 5, told PC Gamer in 2020 that he does not think one unit per tile "works very well, either" and that a stack model is "probably better" for a game of its scope. Johnson wrote in his Designer Notes on Aug 10, 2021 that the "carpet-of-doom" is an ironic reference to Civilization 4's "stacks-of-doom". Civilization 4 vs 5, 6 and 7 compares the systems, and the War Guide puts this page into practice.

Frequently asked questions

How are combat odds calculated in Civ 4? Civilization 4 compares the two units' Strength after all modifiers: equal strengths give each side an even chance and double the Strength gives 2 to 1 odds, per the Civilization 4 manual (p. 40). The game shows the exact odds before you attack.

What is a stack of doom in Civ 4? A stack of doom is one giant pile of units, which Soren Johnson's afterword to the Civilization 4 manual (p. 182) calls a loophole: the best defender in a stack always fights. Siege units' collateral damage was added as the counter.

How does collateral damage work in Civ 4? In Civ 4, Catapults, Cannons, Artillery, Bombers and Battleships damage several units in the defending stack when they attack, and Barrage promotions raise it by up to 50%, per the Civilization 4 manual (pp. 42, 137). Beyond the Sword caps the damage each siege unit can deal.

What is a first strike in Civ 4? A first strike in Civilization 4 is a chance to damage the enemy before it can strike back, canceled if the enemy also has first strikes, per the manual (p. 33).

How much defense does fortifying give in Civ 4? A fortified Civ 4 unit gains +5% defense per turn it stays still, up to +25%, and a Fort gives +25%, per the Civilization 4 manual (p. 41).

Do Walls and culture defense stack in Civ 4? No: a Civ 4 city's defenders get either the Walls and Castles bonus or the cultural defense bonus, whichever is higher, and Musketmen and later units get only the cultural one, per the manual (p. 49).

What terrain gives the best defense in Civ 4? Forest and jungle give +50% defense in Civilization 4 and hills +25%, per the manual's terrain list (pp. 108-110); mounted and siege units get no terrain bonus.