Take out zombies...
	Turret...
	Upgrade...

Make sure you take out the zombies before they get to the object you're protecting, otherwise it will be Game Over before you know it.

Repair your turret as soon as it breaks down. It acts as a barricade as well as a weapon.

Upgrade as soon as you can. More powerful weaponry is essential in the later levels, and makes the earlier ones much easier.