Simplifications that CivIV has are: all unit types have 100hp and there is no difference between attack and defense strength. Now, the changes:
- Unit strength is proportional to current health.
- Firepower depends on combatants' strengthes and for unit of s1 against one of s2 it is equal 20*(3*s1+s2)/(3*s2+s1), thus varies from 6 to 60. This helps to widen the region of not too certain combats. 1.52 patch has replaced actual strengthes here with full hp ones, and 1.62 brought them to the average of the values.
- First strikes are rounds when only one side may suffer damage. Each unit has a number of them, the difference matters in the combat.
- An attacker might withdraw from the combat in case it would lose otherwise. The probability depends on unit type and promotions (in Civ3, on veteranship).
- Instead of veteranship, there is experience/promotion system. Promotions help different game aspects. Some promotions are got another way from type, build or effects; build experience bonus is also posdible. A promoted unit is healed 50% but unfortified. In the commercial game, 41 different promotions exist (some require some another).
- The promotion-giving effects include unit superposition, like presence of a Great General nearby.
- Cities have cultural defend bonus (walls act only against early units), normally growing alongside with city radius. This bonus can be reduced by bombarding the city, then it regenerates.
- Thing closest to the former bombardment is attack with collateral damage; the difference is that there is a specific defender, and the least hp to remain (at a collateraly damageable unit or also the direcly hit one) and maximal number of units to inflict collateral damage depend on the damager. Damager strength is halved for collateral targets.
- All production kinds are produced separtately. If you switch from Warriors to Workers, you produce Workers from 0 shields, but can finish Warriors afterwards from what you have already invested. Obsolete productions are converted to gold automatically.
- Cultural defense, see above.
- Cities have healthiness and unhealthiness counters from terrain, population, buildings etc. (there is a basic health limit, fresh water gives +2 health, each citizen produces 1 unhealth, each flood plain tile produce 0.4 unhealth but a wood tile 0.5 health...). All the population exceeding the difference consumes extra food, and the city can't celebrate with such citizens. This is instead of aqueduct pop caps etc.
- Unhappy citizens don't incite a rebel. They just refuse to work themselves (not related to resistance in a newly conquered city). Celebration rules are the same as in previous games. Free happy city size is adjustable by the difficulcy level.
- Settlers and workers don't cost population. Instead, as long as the city produces such a unit, any its food surplus is converted into shields.
- Also, you can suspend city growing voluntarily with a "Don't grow" check.
- Slavery civic allows hurrying by whipping citizens, like in some Civ3 govs. It works here as well for wonders but with lower pop2shield ratio. Also, the shields are affected by usual production multiplying effects.
- (Probably similar to Civ3) A city has an unhappiness counter. Unlikeable events like whipping a citizen increase it +10 per lost pop, and the simultaneous unhappy faces rise on (counter_value+9)/10. An unique Aztecs' building halves whipping unhappiness increments.
- Cities produce Great People Points. After a city scores next threshold of gpp (increasingly big), a special unit appears that can give the player much advantage. There are 5 categories of GP, which one appears is random but gpp are also of 5 types and the type odds are proportional to the accumulated amount of each one.
- Instead of corruption that reduces your incomes, your growing empire cities suffer increasing expenses summing:
- Distance to the capital component (7 + p)*int(0.29*dist)/8 (halved with courthouse, removed at Communism)
- Scale component, approx. (0.6 + 0.033*(city_pop - 1))*N_cities/2
- Corporative expenses (4g per corp present go to its HQ)
- Colonial expenses (for cities not on one continent with the capital)
- Great people may be converted into super-specialist citizens (actually, mostly work as a building unbuildable another way, even no food consumed). Also, there are free specialists for some buildings (again, may be mostly dubbed bu usual building effect).
- Percentages of religion believers are stored for a city (correction: percentages likely appear only in Civ5, in Civ4 it either is in the city or not).
There are three stadies in the game. In the beginning, barbarians are animals that are generated one by one out of sight of any city or non-barbarian unit, out of 5x5 square around any (non-barbarian?) unit. Animals are different from other barbs in that they can't enter or attack cultural borders. Since a turn, regular barbarians appear the same manner (can be met earlier in huts) that are most like Civ2/Freeciv barbs; rare barbarian uprisings spawn packs of units, like in Civ3 (but no encampments). A while later, barbarian cities appear in no man's lands with the same limitation. Barbarians here are more clever managers then Freeciv ones, basically it's a regular AI minus diplomacy.
- Additionally to connectivity, strategic resources require tile improvements to be accessible.
- Additionally to strategic and luxury resources, there are food resources that increase food and (with a certain improvement) health in connected cities.
- Some resources double the production of some kind.
- Additionally to tiles, you can get some resources from wonders or corporations. E.g. Broadway wonder produces 5 Hit Musicals.