The combat rules explain how to calculate combat strength - but they do not explain what to do with it! Is an attack successful if the attack value is the same as defense, or does it need to be greater?
Also, all of the rules regarding retreating and soldiers refer to an attack and not the result of an attack. As written a soldier which attacks automatically destroys what it attacks irrespective of the defense of the attacked item, and mounds are destroyed when attacked by a worker. I suspect that this is incorrect and is a result of translating the rules into English.
It sounds like you are overlooking this:
If a planned move would end on a hex occupied by an enemy unit or Mound, then it is an attack. This is legal only when the total attack strength (including support) is greater than the defending strength of the defending unit or Mound.