These are my final rules on shields.* As usual, they are adapted from Shields Shall Be Splintered.
All shields improve the wielder's armor class by 1 against attacks coming from an appropriate direction, i.e. the front or the shield flank.
Any shield may be used to absorb the damage from an attack after the damage has been rolled and announced, but the shield is thereby destroyed.
Large shields may be used to absorb the damage of spells, breath weapons, and the like in the same manner. Small shields may not.
Shields of normal construction are no defense whatsoever against ballistae, catapults, trebuchets, cannon, or firearms.
Magic shields usually improve the wielder's armor class by an additional +1, +2, or +3.
Any time a magic shield is used to absorb the damage from an attack, its bonus decreases by one. Once the bonus is reduced to 0, it loses one magical ability (if any) per attack absorbed. Once it has lost all magical properties, it functions as a normal shield.
Magic shields are effective against all weapons. If used to absorb the damage from a siege weapon (including artillery), the shield is destroyed regardless of its degree of enchantment and the defender is thrown clear.
N.B. "Small shields" refers to bucklers and other small handheld shields. "Large shields" refers to heaters, kites, tower shields, round shields, etc.
* Until such time as I grow weary of them.