New combat system
As for combat changes, we're in for some major alterations here. First, units now represented by three men on the map and its health (or toughness) is always the same, i.e. they will never be veteran and elite level. What can change is other parameters like strength which can grow with gaining combat experience. See unit promotions below.
Second, all units are now given a unified strength value instead of attack and defense skill.
Third, each unit falls into one of the three major unit types: melee, mounted or archers. Every unit type has some benefits regarding the others, for example, horsemen have a bonus when attacking archers. This makes combat in Civilization more comprehensive, now you must build and use different types of units otherwise you opponents will simply use special type of units as a countermesure and will be able to effectively block your efforts.
Next, fortify in the field adds less defense bonus than earlier, but it increases with time spent in fortification. Max. defense bonus is 25% and is given after 4 turns standing.
At last, a very complex system of unit promotions was created. For every battle a unit wins it gains a certain amount of combat experience and after some predefined number is accumulated this unit "makes a level" and becomes slightly tougher. You have several dozen of skills to develop, from strength and formation (better protection against mounted units) to accuracy and garrison (better defense when protecting a city).