For the players who love this game but find it too easy try imposing challenges on your self. I just finished a pure airforce playthrough with no additional batteries or fortresses. Torpedo bombers, Helicopters and Bombers are surprisingly effective if previously you relied on spamming destroyers for the backbone of your forces.
 "Black Navy War"){kind=icon}
 "WereBox 2"){kind=icon}
 "Creeper World: User Space"){kind=icon}
 "Replaceable Blocks"){kind=icon}