The Knight is a melee class. As of the latest version of Castle Story, they have been replaced with the Knight Weapon Kit, but the kit and the unit are similar. They have high endurance, making them useful for screening much more vulnerable Archers or civilian Bricktrons from Corruptrons. However, they also have low damage output, and as such will eventually lose to a Corruptron in a one-on-one fight.
As of the Steam Early Access build, the Knight class is now named the Warrior class.
Warriors have the highest health of any Bricktron class, enabling them to endure prolonged engagements, but their low strength means they need support to survive. Effectively, they are the inverse of the Archer, and the two classes compliment each other well. Players should use Warriors to pin hostiles in place, then use Archers to defeat the now helpless foe.
A player should be wary of spawning too many Warriors without Archers to support them, as Warriors need a 2-1 numeric advantage to defeat Corruptrons, and the rate of increase in Corruptron spawns in Survival Mode will quickly make such numbers impossible to maintain.
Following the renaming of Knights to Warriors, the idea has been tossed around that Sauropod may intend to implement a Knight class as a potential upgrade to the Warrior class in a later build. What effects such an upgrade would have are unknown.