Combat Bonuses
The tile a battle is fought on gives the DEFENDING side a defensive bonus. It works as extra armor with no cap: every hit the defenders take is divided by (1 + 1.2 x bonus), while their own hits are unchanged. Against attackers of their own kind, defenders trade about 2 for 1 at +100%, 3 for 1 at +200% and 4 for 1 at +300%. An attacker's armor penetration shrinks the bonus it fights against. At 40% penetration, +100% counts as +60%. The attackers never get it. The bonuses below add together.
Terrain
- Arid Hills: +10%
- Arid Mountains: +25%
- Desert Hills: +10%
- Desert Mountains: +25%
- Forest: +10%
- Frozen Hills: +10%
- Frozen Mountains: +25%
- Grassland Hills: +10%
- Grassland Mountain: +25%
- Jungle: +10%
- Snow Hills: +10%
- Snow Mountain: +25%
- Tundra Hills: +10%
- Tundra Mountain: +25%
- Water: +25%
Improvements
- Bridge: +25% once complete (a pillaged one gives nothing)
Cities
- A city gives its defenders +20% for every culture level, counting a new city as level 1, up to +100% at level 5.
- Sun Tzu: Adds +15% defensive bonus when defending this city.
- Kofun Iron Cuirass: Adds +20% defensive bonus when defending this city.
- Tizona: Adds +25% defensive bonus when defending this city.
- Castle: Adds +50% defensive bonus when defending this city.
- City Walls: Adds +25% defensive bonus when defending this city.
- Motes: Half the defensive bonus of City Walls in every city, walled or not
- Bombardment wears a city's whole bonus down, culture included. It regrows 5% of the full bonus per turn after 5 quiet turns. A captured city's defense falls to nothing and regrows from there, even when it is liberated.
- Each bombard also wounds every unit in the city for 50% of the defense it took, as a share of the unit's full health, never below 1 hitpoint. A siege unit gets one fight a turn. It cannot bombard after moving or attacking, and a stack cannot attack after one of its units has fired.
Fortification
A fortified stack gives every unit on its tile +10%. Order a unit to Fortify and it digs in until it is moved.
Empire-wide modifiers
Wonders, policies, governments, civilization traits and artifacts can change unit stats for the whole empire. Some only apply inside or outside your own borders.
- Britain: +1 Movement for naval units
- Mongolia: +1 Movement for land units outside your borders
- Lakota Nation: +1 Movement for land units inside your borders
- Home Guard: Your units gain 15% defense when fighting inside your cultural borders.
- Professional Army: Your units gain 10% defense.
- Sparabara: +2 Ranged Damage for Archers
- Thalassocracy: +2 Melee Damage for Naval units
- Thalassocracy: +2 Ranged Damage for Naval units
- Warrior Code: +2 Melee
- Chichen Itza: Your units gain 10% melee attack when fighting outside your cultural borders.
- The Great Lighthouse: Adds 2 movement to all your naval units.
- Machu Picchu: Your units gain 25% defense when fighting inside your cultural borders.
Armor penetration
A unit's armor penetration is subtracted from the defender's armor before damage is reduced. 10% penetration against 25% armor leaves 15%. It can never turn armor negative.