[This article is an expansion of Shield Defense.]
Magic shields have benefits beyond the increased improvement to armor class. Although "shields of normal construction are no defense whatsoever against ballistae, catapults, trebuchets, cannon, or firearms," magic shields are. Against all weapons, magic shields confer their armor class bonus. All missiles except cannonballs can be blocked by magic shields (if the saving throw is successful). Magic shields can be sacrificed to defend against any damage-dealing area effect attack or weapon (including cannon). (In the case of cannon, the shield is destroyed and the defender is thrown clear.)
Optionally, a magic shield may be sacrificed twice. The first sacrifice nullifies its enchantment. The second sacrifice destroys the shield.