Hmm, I think focusing on defensive structures is a bad direction for the game to take. StarCraft is definitely one of those RTS titles where the phrase "the best defense is a good offense" really shines.
The correct unit composition should always be favoured over defensive structures. 'Cos once you've pushed back the Mutalisks, you can't lift up your Missile Turrets and attack the Zerg base with them.
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