Defense

Defense is flat subtraction taken off each individual hit. It runs after the random damage roll and before any percentage damage reduction. No hit can be pushed below 1, so against a swarm of weak, fast attackers a large defense stat effectively pins each of them at a single point of damage rather than zeroing them out.

Because it works per hit and not per second, defense is worth the most against many small attacks and the least against one enormous one. That is the opposite of percentage reduction, which is why endgame characters carry both.

How much each point is worth depends on the world. Classic subtracts half your defense, Expert three quarters, Master the full value. That reads as defense getting better on higher difficulties, and technically it is, but enemy damage climbs faster than your protection does.

Work it through with 50 defense against a Giant Bat. In Classic its 45 damage drops to 20 after your 25 point cut. In Expert the same bat swings for 90, your 50 defense only removes 38, and you eat 52. The incoming number doubled while your protection improved by half, so the hit lands for more than twice as much. Stacking defense on higher difficulties is still correct, it just will not hold the line the way it does in Classic, and percentage reduction plus simply not being hit carry more of the weight.

One quirk shows up in PvP. Set bonuses that trigger on a damage threshold, such as Beetle Might charging or the Spectre Hood draining life, evaluate the hit as though the world were Classic no matter what difficulty you are on.

Enemies get no difficulty scaling. In every world an enemy removes half its defense from your hit, rounded down, and that subtraction is applied to each hit separately. A weapon that lands twenty small hits per second loses far more to an armored target than one that lands two heavy ones, even when their listed damage per second matches.

This is also why critical strikes and defense interact badly. The doubling happens after the subtraction, so on a heavily armored target a critical strike doubles a reduced number.

Armor penetration is a hidden stat that hands back part of what the enemy's defense takes. If your total meets or exceeds the target's defense, the whole subtraction is canceled and you deal full damage. If it falls short, you recover half of your penetration value instead, so partial coverage gives partial benefit.

The practical limits: it does nothing in PvP, nothing against a target with zero or negative defense, and nothing above the highest defense value you actually fight. Several debuffs supply it against the affected enemy, and some projectiles carry their own value that adds to yours on top.

Damage over time from debuffs ignores defense completely. So does drowning. Lava does not: lava damage runs through defense like any other hit, which is why a well-armored character can wade briefly through a pool that would kill a fresh one, and why lava-immunity accessories matter less once your armor is deep.

Most of your total comes from armor, and completing a matched set adds its bonus on top, so mixing a helmet from one tier with a body from another usually loses more than the individual pieces gain. After that, the reliable additions are the shield accessory line and the Warding modifier, which is cheap to reforge and works on accessories that grant no defense of their own.

Pre-Hardmode peaks in the mid seventies: the Molten Helmet set, the Obsidian Shield chain, an Obsidian Skull, a Shackle, Warding on everything, then Ironskin, a well fed buff and the Dryad's blessing on top. Expert and Master add slots, which is where the last few points come from.

Hardmode roughly doubles that ceiling. The frame is Solar Flare armor or a Hallowed Mask with the heavier shields, Flesh Knuckles, a Berserker's Glove and a Celestial Shell, which only pays its full defense at night while you are out of the water. An Aegis Fruit is a permanent addition worth grabbing the moment it drops, and holding a guard with the Brand of the Inferno adds more while the block is up.