3-layer military defense! Air (top strip), Ground (middle), Underground (bottom).
Towers: AA Gun (air), Artillery (ground AoE), Minefield (underground), SAM (air homing), Bunker (ground+infantry). 3 tiers each.
Support: Airstrike (click target), Reinforcement (spawn soldiers), Artillery Barrage (full-map AoE).
Enemies: Infantry, Tanks, Helicopters, Paratroopers (air->ground drop), Tunnellers (underground emerge), Bombers.
Terrain: Forests (slow), Rivers (block, need bridges), Hills (bonus). Towers only on ground layer.