This is a simple system to replace Armor Class with the more abstract Defense used in other d20 game systems. Basically, all characters gain a base Defense based on the Defense feats they have and their character level. The end result of these rules are characters less dependant on armor and protection items than your standard AD&D characters. Likewise, characters unable to wear armor without penalty, such as Sorcerors and Monks, are harder to hit than their counterparts using the traditional Armor Class system. Naturally the DM will need to make the presense of armor and protection items far more rare to maintain proper game balance.

There are three Defense feats, Light Defense, Medium Defense, and Heavy Defense, each coorisponding directly to an Armor Proficiency feat. Classes who begin with Armor Proficiency feats instead gain the coorisponding Defense feat. These feats have the same prerequisites as the coorisponding Armor Proficiency feat. All references to Armor Class bonuses are changed to refer to Defense Bonus instead. The base DC to hit a character is 10 plus their Defense modifier, as listed below:

While gaining the benefit of Defense, a character suffers certain penalties. Certain physical actions are hindered, movement is reduced, and arcane spellcasting has a chance of failure, as the character is having to divert their attention from such activities to properly defend their self.

* May not run

The skills that suffer penalty for using Defense are Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand, and Tumble.

Characters with multiple Defense feats must announce which Defense they are using at the beginning of an encounter, but may switch this decision during of their action, though they may not use a Defense which restricts their movement more than the distance they have moved in this round.

Characters seeking more protection may still don armor. There are three types of armor, Light, Medium, and Heavy. Each provides Defense bonuses but also places further penalties on specific Skill Checks as well as arcane spellcasting and Dexterity modifiers to Defense.

Characters may also use shields and helms to further augment their Defense. Shields provide identical bonuses to those listed in the Player's Handbook. Helms function exactly as shields, and cost a similar amount, but apply twice their Check Penalty to all Listen and Spot checks in addition to the standard penalized Skills.

The following character classes begin play with Light Defense, Medium Defense, Heavy Defense, and Shield Proficiency:

Cleric, Fighter, Paladin; Aristocrat, Warrior

Barbarian, Bard, Druid, Ranger

Rogue; Expert

Monk, Sorceror, Wizard; Adept, Commoner

Barbarians using Heavy Defense may not use their increased movement bonus. Druids must use wooden versions of any armor, or shields they don. Monks who use Light, Medium, or Heavy Defense lose their Wisdom bonus to Defense, their Defense bonus for level, better rate of unarmed attack, increased movement, and suffer the arcane spell failure chance on their supernatural abilities.

Naturally these rules provide a significant shift to the standard AD&D game, and should be carefully considered before implimenting in your campaign.