The enemy is approaching your fortress and you are the first and last line of defense. Be prepared for an Armed Invasion!
Some Tips & Tricks:
Save Yer Ammo - Your ammo regenerates slowly at first, so make every shot count.
I Heart Turrets - No matter how good a shot you are, three guns are better than one. Save up for your turrets and buy them as soon as you can - they will save your life!