From DDO Compendium


Critical hits and sneak attacks produce higher damage and can be deadly. Fortification represents your chance of downgrading these types of attacks to normal attacks instead. Fortification only helps you against critical hits and sneak attacks.


When you are hit by an attack that is a critical hit, or one that deals sneak attack damage, you have a chance to reduce the damage you take from that attack equal to your fortification - the target's fortification bypass.
  • Critical Hits. When fortification reduces the damage from a critical hit you take damage as if the attack's critical multiplier were x1. In effect, taking the attack's normal damage.
  • Sneak Attacks. When fortification reduces the damage from an attack that deals extra sneak attack damage, the sneak attack damage is completely negated. You still take the attack's normal damage.
If your fortification is greater than your attacker's fortification bypass + 100, you are effectively immune to critical hits and sneak attacks from that attacker.