Fortify your base with several turrets and start blasting the hell out off the enemy aerial units. Choose your weaponry wisely as each has its own advantages and disadvantages.

They’ll try to overrun your defenses, so you better have an accurate aim and blast them from the sky.

Blast the oncoming enemy units out of the sky.

Use the mouse to aim the selected turret, left click and hold to set power, release to fire.
In between waves you can buy and repair turrets.