Placement matters for a couple of things:
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Tank: Your heavy(est) defense is usually in tank/center position, as that position draws four lanes of fire, not one apiece as the outside positions do.
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Left to right: Game AI fires left to right, so you want your healers, resurrecters, etc on the far left. (AI is used on war defenses, not attack, etc).
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Threes: Some hero’s specials cover themselves and two nearby allies; you don’t want these heroes in the far corners, or some of the special’s potential coverage is lost.
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Slow: You might also not want slow heroes in the corner in defense as it will take them longer to be hit, to charge, and to fire.
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Next to: Good idea not to have two heroes of the same color next to each other, as it’s easier for the enemy to damage both of them with the same hits.