The core gameplay of Kingdom Invasion is that of a flanking system. The map is built on a hexgrid - similar to that of the civilization series - with enemies and obstacles dotted around it.
If an enemy spots you, they will move to attack and damage you. Naturally, you can fight back. This is where the flanking system takes effect.
Attacking an enemy head-on will deal minimal damage, and will give them ample opportunity to attack you in return.
Attacking from the front, but at an angle, will deal a slightly increased amount of damage, and an even greater amount of damage will be dealt if attacking the enemy's rear at an angle.
Finally, the greatest amount of damage, that of flanking and attacking from directly behind, will be the most effective way of defeating enemies. A lot of the time without them being able to respond. But be warned! Some may be resistant to these attacks and not fall immediately!
The picture below demonstrates the flanking system, with the numbers representing the amount of damage dealt.
Controls:
W - Directly forwards
Q - Forwards to the left
E - Forwards to the right
S - Directly backwards
A - Backwards to the left
D - Backwards to the right