Hmmm... I like this. And no, it isn't too difficult, in my opinion.
My views about things:
- You'll work with mana, which will either generate, or has to be gathered.
- You have schools... This can either mean you have to buy different types of wizards fo different types of spells, or send them to school.
- Each spell goes two ways. You can use it as a spell in your advantage (as a defense, for example), or as a spell in the other's disadvantage (as an attack, for example).
Now, I do have one remark.
Light: Warmth inside slowly regenerates-heals units at 3+X HP/sec
Dark: Heat insides slowly burns-damages units at 3+X HP/sec
Doesn't that kinda mean it does 0 damage? Or do you mean by Light and Dark the effects it has in Day/Night?