Fortress Knight

Focused around heavy armor, this class builds on its heavy defense with powerful endurance and resistances. 
The fortress knight is a self sustaining terror on the battlefield, easily able to weather multiple blows from even the most powerful of enemies. A dragon might expect to tear a measly humanoid in half, only to find their claws stopped by a dense ball of iron and flesh, giving the rest of the party time to escape, or slay the beast.

Dense Physique

When you take this subclass at 3rd level your martial training veers in the direction of fortitude, rather than aggressive combat. Your hit point maximum increases by double your fighter level, as well as whenever you level up.

Rooted Stance

At 7th level your martial training allows you shrug off physical blows like insubstantial mist. While wearing heavy armor you gain resistance to bludgeoning, piercing, slashing, and force damage. Additionally you have advantage on Strength saving throws made to avoid being pushed, pulled, or knocked prone.

Iron Body

Starting at 10th level, you have the ability to preternaturally strengthen your body against malicious forces. Three times per long rest, you can choose to double your proficiency modifier for Strength and Constitution saving throws. This can be done after the roll is made, but before the DM tells you if you succeed.

If you are not already proficient in Strength and Constitution saving throws, you become proficient in them.

Impenetrable

Starting at 15th level, you can strengthen your body, even while not wearing armor. As a reaction to being struck by an attack, you can add your proficiency bonus to your AC. This lasts until the start of your next turn.

Impregnable

At 17th level you have taken more hits than any creature still living. You may choose from a list of damage and condition immunities, becoming the ultimate in defensive force.

Acrid Shield

You become immune to all manner of foul biotoxins and acids. You gain immunity to Acid and Poison damage. By virtue of this, you also become immune to the poisoned condition.

Concussive Shield

Thunderous smites and detonations now glance off of you like water in a storm. You gain immunity to Force and Thunder damage.

Eclipse Ward

The forces of life and death hold little sway over you. You gain immunity to Necrotic and Radiant damage. You also become immune to being blinded.

Frigid Bulwark

Your body becomes hardier than stone. You gain immunity to Cold and Bludgeoning damage.

Plasmic Coil

Superheated materials no longer affect you, allowing you to plunge your hands into a forge and come out unscathed. You gain immunity to Fire and Lightning damage.

Psychic Schism

Your mind is as sturdy as your body, granting you a myriad of mental defenses. You gain immunity to Psychic damage. Additionally, you become immune to being charmed and frightened.

Rampart of Deities

Swords and spears break upon your flesh, you are the ultimate in melee defense. You gain immunity to Piercing and Slashing damage.