Knight's Shields can be acquired in a variety of ways. First off they can be dropped loot from any Knight you kill in World 1 (Boletaria Castle) so long as the knight is actually using a shield, later in 1-3 they can be purchased from the Dregling Merchant, and lastly there is one that can be found on a corpse in 1-4. The Knight Shield is a notable item because if it is fully upgraded to 10 it has the highest Hit Resistance of any standard size shield in the game. If you take it along the "Dark" upgrade route it will never defeat the magic defense of the Dark Silver Shield but it will still have superior Hit Resistance. Hit Resistance is important because it is what determines how much Stamina is needed for a successful block and likewise how easy it is to break your guard causing you to be "stunned". Standard size shields also have an advantage in that they can be used to parry, something large shields can not do.