Shields are indestructible and can not be broken. However, you can strike around them or tire your opponent to the point of dropping their defense.
A somewhat cheap, but effective, way of getting around a knight's shield is to jump and perform an overhead swing. Unless the defender looks up to raise their shield, the strike will often get over the shield's hit box. Another option is to side step while swinging from that side. For example, step left while swinging from left to right. They'll have to work to keep their shield in position to block your strike.
Also consider the fact that they lose a good chunk of stamina when blocking a strike with their shield. If you continue to hammer away while they turtle, their stamina will drain. If they block with no stamina, they'll falter leaving an opening for you to strike.
An extra way to get past a shield is to use a kick. If a kick connects with a shield, it will always knock back the shield user and stagger them. A fast weapon is required to take advantage of the gap where the knight is staggered from the kick.