A big characteristic difference between 100 Wars and other popular tactical games today is that you don't get an advantage just from being the first to attack. A melee unit attacking another of the same melee unit out on an open field will result in roughly equal casualties on both sides. For this reason, you must use units in concert strategically, use terrain, and abilities all to gain an advantage.
Every time a unit attacks another unit, each individual inside the unit will roll according to Wound & Kill Hit% to determine how much damage is done to the other side.
Wound Hit% = 30 + ((Your Attack - Enemy Defense) * 5) (Result cannot be lower than 5)
Kill Hit% = 10 + ((Your Attack - Enemy Defense) * 2)
Zone of Control is a feature that limits movement of a unit around an enemy unit to one move point. Please see this page for a deeper explanation of Zone of Control, the Supporting Effect, and Surrounding: 
Also see MORALE section for more.