For a great and relatively sheap defence on bridges, put a spec ops witha mine in front of it and a stealthed sniper behind it, this is great protection for any land unit. If a tank/half track attacks they will hit the mine, the spec ops can attck with rocket launcher from a distance and the sniper can finish it off. the sniper will take 8-10 damage of any infrantry unit and you can use the spec ops to finish the attack if necessary.
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