Putting arcane casters in medium armour is min-maxer BS, though. We're talking half-plate. Basically the idea seems to be that your arcane super-nerd decided to suddenly go out into the yard and play in the mud with the squires to learn how to move and jump and roll and parry and dodge while wearing fairly heavy armour.
Why not insist that your character is actually from planet Krypton while you're at it?
