Like previous Heavy Cavalry units, knights are the mounted front runners among all troops. Their default behavior is to seek and destroy defenses. With a large amount of hitpoints, they can endure more than any other troop of their age, as well as dealing a lot of damage before falling. However, their disadvantages is that they have low damage and can be killed easily by Ballista Towers.
"A knight served his sovereign or feudal lord as a soldier on horseback. At the height of the Middle Ages, Knights ruled the battlefield. They wore the heaviest armor, rode powerful horses, and carried swords (or other melee weapons) and long, deadly lances. A company of knights at full charge was a terrifying spectacle, especially for the target. If the defenders held firm they could often drive away the attackers, but if they wavered or broke, they would be torn to pieces by the rampaging knights."
Knights have a lot of health but are slow and have low attack power, so use them in groups.
Knights are weak against many defenses grouped together.
Due to their high health, Knights can be use as meat shields to protect infantry from the defenses.
Ballista Towers can destroy Knights easily like all heavy cavalry. If you can, try swarming it with heavy infantry or ranged infantry to destroy it quick enough since the ballista tower is slow.
When pushing for trophies, you would need to destroy the Town Center. In many bases, the town center would be in the center of the base. First, train up an army of knights and a wall breacher such as a veteran wall miner. When attacking, deploy your veteran wall miner and wait till it destroys the necessary walls. Then, deploy all your knights in the areas which the Town Center is more close to. If there are a few defenses blocking the way, destroy them. Then rally your knights to destroy the Town Center.
Knights would ignore defensive troops and would pursue their targets which are defensive buildings. However, if a defensive troop attacks the cataphract, the knight will attack back until the defensive troop is killed; thus making the knight distracted. Try to use and deploy ranged infantry such as crossbowmen or heavy infantry such as pikemen after deploying the knights to eliminate the defensive troops.
Knights are a good alliance troop due to its high health; making it longer to kill them.
The Reinforcements technology in the Library spawns knights in the forest and they attack troops of the attacker.
Knights would oftenly use swords on horseback or dismounted. Shown in the image above were a knights in several styles using a polearm weapon which is probably a spear. Knights with spears are really rare and they use lances (which is a type of spear) for jousting; which is a Medieval martial game.