Now, about the game system :
Overview of the combat system
During the tactical battles, each side will play all of its characters at once. Each character have 2 actions, like in Firaxis' XCOM (yes, I know...), or Descent: Journeys in the dark. Attacking ends the activation of the character (so only one attack per round).
Combat takes place on a hexagonal map.
Some abilities can allow a character to do a bit more (like moving 2 hex, and attack twice in melee at the cost of 2 actions for instance).
Elevation gives an attack and defense bonus to the highest character.
Facing does not matter in the game, but flanking does provide an advantage when attacking.
Flanking
The red character on the left is flanked because there is no way he could not have someone in the 3 rear hexagons, regardless of his facing.
The other two red characters are not flanked because the middle one would have no one in the back if facing the hexagon between his two opponents, and the one in the middle is facing his 3 opponents, and have no one in the back.
Ignoring opponents
An important consideration is whether one character has been attacked during this turn. Characters who have not been attacked hampers adjacent opponents when they try to perform an attack. They also provide a higher flanking bonus to allies that manage to perform an attack on a target adjacent to them. They also have a stronger free attack against opponents that try to disengage than characters that have already been attacked this turn.
This was done to de emphasize focus firing, and give a purpose to weaker opponents.
Disengagement and Zones of Control
A character already has some kind of zone of control through the threat of backstab, and defensive support. However, they also have other effects on adjacent hexagons :
moving from a hexagon in the zone of control of an opponent costs a movement penalty, and may grant him a free attack.
Someone trying to disengage will give a free attack to an adjacent character who has not used his attack, or who has already attacked him.
In this exemple, Mr HammernChainmail can get a flanking position against the red knight of Libra : he can either make a detour, or pay 1 extra movement to get from one ZoC hexagon to the next. But if he does so, he will give a free attack to the libra knight, unless he already attacked someone else during his turn.
Attack is an opposed roll (attack vs defense), and the margin of success of the attacker will impact the damage he will cause.