Combat is a simple process decided by the units’ Attack and Defence values, how good you are at rolling dice, and the number of models in each unit:
1) If your unit is still over half its starting number of models, it fights a lot better (with 12 dice) than if the unit is at half strength or less (rolling only 6 dice).
2) The attacking unit scores a hit for every dice that equals or betters their Attack value, and the unit that has been attacked scores a hit for every dice that equals or betters their Defence value. Simple.
3) The number of hits is compared to the target’s Armour rating to work out how many casualties are taken.
4) Units taking casualties need to check their Courage to find out whether they stand, retreat, or rout from the battlefield.
Differentiating between offensive and defensive capabilities allows for variation in how each troop type performs: for example, Foot Serjeants have an Attack value of 5+ and a Defence value of 4+, meaning they’re actually deadlier when the enemy comes to them (using a hedge of spears for protection); Fierce Foot on the other hand have a much better Attack value than Defence value, so you’ll want them to be charging at their enemies rather than being charged at themselves.