The Knight is the fast unit of High Middle Ages. It has moderate damage and defense. It is effective against the ranged unit class but is vulnerable against the light class.
It is worth noting that at this point, Fast units start becoming a decent unit for stats. They have enough attack and defense to be used regularly. They also have a special ability, Chivalry, which gives them a +3 boost in both Attack and Defense only if it is the only unit with Chivalry (i.e. Knight and Heavy Knight) in army.
Knights should be used singularly in battles, to allow for Chivalry to take effect. Having multiple Knights requires all but one Knight to die before Chivalry takes effect.
Knights should be used to charge into enemy lines and kill ranged and artillery units.
Against Same Age Units
The knight is easily killed by the berserker and should stay away from them, unless there are no other types of units to attack.
Knights are super effective against the crossbowman as they gain a bonus against them. However, since the crossbowman is a ranged unit, crossbowmen don't fear retaliation damage.
Knights are somewhat effective against the heavy infantry as neither of them gain combat bonuses against one another. The Heavy Infantry, however, gains a bonus 6 defence on plains, while knights have no terrain bonuses.
Knights are super effective against the trebuchet as the knight can reach the other side of the battlefield in two turns, nullifying the trebuchet's effectiveness. As a bonus, knights can also two hit KO a trebuchet. Despite this, some defences use Berserkers to protect the trebuchets from knights.