Having two units that are slow and none that can fly (I never got to the endgame) + archers only at level 1 with a penalty to their ranged attack = storming the Academy's Castle becomes a lottery. I'm not suggesting adding another uber unit to the army, but just introducing a mid-level flying unit, which, in my opinion, wouldn't break the balance.

However, introducing a powerful level 6 flying unit into the army would seriously disrupt the balance, in my opinion.