Anti Air

Anti-air system is introduced after the air system.

Levels
Lv 1 Anti Air: shoot down 10, cost 1B

Lv 2 Anti Air: shoot down 20, cost 2.5B

Lv 3 Anti Air: Shoot down 30, cost 4B

Suggestions

 * Airports
 * Cities
 * Large Army Groups
 * Weak Army, e.g. ranged