[Rytek]

2 different things. They do not add.

Defence of the castle is the strength of the walls. There is a formula given in the manual on how armies tear down castles. Easiest way to remember it is basically take how many troops are inside the fortress and subtract their number from the sieging army. If the sieging army is bigger, it will do damage to the fortress. In the case of a def 100 fortress, the army needs to do 100 points of damage to the wall thru the siege. Now, the formula is more complicated than what I described its something like taking str of troops, flying ability (bonus) animal or mindless ability (penalty) in to affect.