Battle Abilities
Some units carry a battle ability that gives them an extra part to play in a fight. A unit's page lists its abilities.
Flanking
In the flanking phase a flanker makes an extra melee attack against a random enemy in the back row (or the middle row, or the front row, when the rows behind are empty). It is how cavalry reaches the archers sheltering behind a shield wall.
Units that flank
Flanking (No Counter)
Flanks exactly as Flanking does, but the flank is never answered. No Counter-Flanking unit strikes back at it. Spear walls still hit it harder in melee.
Units that flank without a counter
Counter-Flanking
When a unit is flanked, the nearest defender with Counter-Flanking (the flanked unit itself, if it has the ability and survived the blow) immediately strikes back at the flanker with a melee attack worth 1.5x its normal roll.
A unit with Counter-Flanking is also a spear wall: every melee attack it makes against a unit with Flanking rolls 1.5x its normal damage, in the front row as much as when answering a flank. Spears beat horses. Horses beat the clubmen and archers that spears do not protect.