The Opal Empire
Open Game Content on this page is designated by a green box, like this one.
The Opal Empire isn’t a medieval setting. Heavy metal armor here is extremely rare (and difficult to maneuver in). Most everyone wears simple clothing.
Nonetheless, the rules designed certain classes with a high armor class (AC) and those classes still have a high armor class in the Opal Empire, just without the actual armor. Replace armor with martial arts defense skills (blocking, parrying, and dodging), mystical protection (second sight to avoid attacks, a wall of wind deflecting attacks), and swirling clothing providing defense.
For example, if you play a fighter, expect to have the AC you’d normally have (15 for heavy armor) without having to wear steel plate. You might just wear robes and maybe embellish it with some leather or metal arm bands for blocking. You might leap over attacks aimed at your midsection and swirl your robes dramatically to repel blows. Of course, a wizard can do these same things — just not as well as a fighter or paladin. Hence the lower armor class for wizards.
Instead of a shield, you have a free hand to block more attacks. That gives you a +1 AC but you cannot use two-handed weapons.