Do not be afraid to enter the town which is full of zombies. At the first sight it might seem that it will be hard task to accomplish, but once you have the proper weapon and armor, you will definitely change the mind. It is so important to buy units, those people will help you to shoot the zombies that are coming along the way to your basement. Remember to scale up the space that you own. It costs a lot to construct the buildings, but they increase the defensive points of you. Consider attacking them first, because it gives you the chance of winning.