The best magic is that of Haven:)
As it was said: "if the enemy cannot get close enough to attack, then they cannot attack. Simple physics."
And meanwhile, you keep hitting enemies with spells and shots:) Sieging is generally very simple - displacement removes archers from the towers; if they climb down, they get a bolt from the djinns and another displacement from the hero; mages poison everyone alive, and the djinns spend their free turns creating illusions:)