I have read the guide and it says the defence is at 80% strength.
Lets say orc A sends 5000 ogres, 500 war horses.
which gives him: 40 500 raw offence, 10 000 defence raw
80% x 10 000 = 8 000 def
So u send 1000 ogres to ambush him, and add 4% more to ensure success?
