Combat Modifiers
There are three types of combat - melee, magic and missile. This section breaks them down and summarizes the modifiers by combat type.  Modifiers are specific to an attribute.  Saying a unit attack is doubled means the attack attribute for that type of combat (magic, missile or terrain in the case of melee combat ) doubled.  The defenders attack (attribute) is always referred to as the Counter Attack.

Melee Combat
Defenders counter attack is tripled.
Attackers have any major unit in stack-top three minor units attack tripled.
Major Unit with the highest combat bonus has said bonus added to all attacking minor units.
Strength adds 10 points to an attacking or counter attacking unit.
A minor unit's moral is always added in melee attack and counter attack.

Missile Combat
Attackers attack cut in half if defender is in a forest.
Attackers attack divided by three if defender is in the mountains.
Attacker attack divided by four if defender is in a city.
Flaming arrow spell doubles attack and counter attack. 

Magic Combat
At this point there are no magic combat modifiers.