These Fighters have learned how to protect others by using their own skills with defense for other people.  The more advanced defenders even learn how to use their defense as an attack, and how to turn their opponents failures against them.

Shield Expertise

At 3rd level, you can use your reaction to cause your shield to give you an additional +1 bonus to AC until the end of your next turn.  This feature can be used a number of times equal to your Strength modifier before a long rest.  At 9th level, the bonus to AC increases to +2.

Shield Offensive

Starting at 3rd level, you can make an attack with your shield as an action.  You have proficiency with this attack.  On a hit, the shield does 1d4 bludgeoning damage, and the target must make a Strength saving throw (the DC equals 8 + your proficiency bonus + your Strength modifier).  On a failure, you can push the target back up to 5 feet.  At 10th level, your shield attack instead does 1d6 damage.

Shield Technique

At 7th level, you can don your shield as a reaction when targeted by any attack.  In addition, if a creature misses you with a melee attack while you're wearing a shield, you can use your reaction to impose disadvantage on their next attack roll that turn.

Deflective Crouch

At 10th level, if you're wearing a shield, you can spend your reaction when a ranged weapon attack targets you to cause it to miss.  You then have disadvantage on the first attack you make on your next turn.

Solid Stance

At 15th level, you can use a bonus action to fall into a defensive stance.  While in this stance, you gain resistance to nonmagical bludgeoning, piercing, and slashing damage, your speed is halved, and you can't make opportunity attacks.  You can come out of this stance with another bonus action.  In addition, you have advantage on saving throws against being pushed or knocked prone.

Disarming Strike

At 18th level, if you hit a creature with your shield attack, instead of pushing them back, you can force them to make a Strength (Athletics) or Dexterity (Acrobatics) check contested against your Strength (Athletics).  If you win the contest, you impose disadvantage on all attack rolls on their next turn.  If the creature is wielding a melee weapon, you can instead disarm them, sending the weapon skittering 15 ft. away in a direction of your choice.  In addition, when you hit a creature with your shield attack, instead of pushing them, you can choose to knock them prone on a failed save.