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If the hero is already an archmage (even if one of the magic skills isn't fully developed), they don't really need strong defense. For example, if they have Chaos magic, Inferno (or Fireball) will eliminate all dangerous units. Even if we're talking specifically about Haven, there are still plenty of options. I won't go into detail about hypnosis and forgetfulness, etc., but I will talk about illusions. They will be the archmage's defense.