I've been thinking that magic attacks should go against something different than the units armor class. Maybe Have a different AC for magic attacks. Armor clad warriors still can't do much against a fireball.
Problem with that is, itd be lower than most pysical def...which will make the warrior/pally a little too easy to kill when they already have not nmuch of an unbuffed offense.
This is also discussed in: https://heromages.com/forums/index.php?topic=62.0
Our current feeling is that two different Defense values would detract from the simplicty of the mechanics that many players find to be a distintictive and refreshing quality of Hero Mages. In the prototype/board game, we had a Mind value (mental defense) that worked similar to this, but it confused a lot of people. Hence, we went down to having a single Defense value.