Defense

Defense is a damage mitigation stat, that you can build through armor, shields, and hero skill trees.

Defense allows your heroes the ability to avoid an enemy's attack. When the enemy attacks you, there will be lower hit rate if your defense is higher.

Items with the highest defense
Combined with Fighter's base defense stat of 5, and the passive skill Ironskin which gives 6 defense, the current highest possible defense is 91, as there is no defense cap.

Skills that lower enemy's defense
Summoner's Curse of Agony lowers defense by 5% to all enemies in range, and Dire Curse which adds the effect by 3% per skill point spent on it.

Fire Mage's Spell Penetration makes attacks done by Fire Mage ignore 2 points of enemy defense per skill point spent on it, with a maximum of -6 defense to the target.

Skills that lower defense
Legendary Hero's Rage, decreases defense by 38% for 11 seconds.

Barbarian's Rage decreases defense by 40% for 10 seconds.

Skills that raise defense
Fighter's Ironskin increases defense by 2 per skill point spent on it, with a maximum of +6 defense.

Legendary Hero's Unyielding Fury adds 3% defense when enraged per skill point spent on it, with a maximum of +12% defense, but even so, as using "Rage" decreases the user's defense by 38%, it is really just lowering the defense drop by 3% per skill point.

Barbarian's Unyielding Fury adds 5% defense when enraged per skill point spent on it, with a maximum of +15% defense, but even so, as using "Rage" decreases the user's defense by 40%, it is really just lowering the defense drop by 5% per skill point.

Archer's Nimble Dodge increases defense by 2 per skill point spent on it, with a maximum of +6 defense.

Cleric's Holy Shield increases defense by 2 per skill point spent on it, with a maximum of +6 defense.

Cleric's Healing Shield increases defense by 2 on the healing target per skill point spent on it, with a maximum of +6 defense on the target.

Tree Boss
I don't know how, but the defense of the tree boss could increase during the battle.

Miss  Miss  Miss  Miss ....

Even the tree demon in siege mode has this effect after casting the 'song' spell. It's suppose to grant posion immunity, but is actually damage immunity. Ironically, most of the shield walls on the other minions don't do a thing.