hey how old is your Dobermann? Most Dobermann's have the guarding instinct bred into them - they get more and more watchful as they mature and become more territorial. This might take up to 2 years of age. If your dog is younger than this, just give him a little more time boss. Secondly, for your dog to guard more effectively it has to feel that your entire home and gounds are its own territory which it will defend against all intruders. For this, you need to let it roam free and get acquainted with every nook and corner of you house. Love for you will make him a much better defender and guard than being tied up all the time and resenting it. However, one thing I can suggest to make him a better guard dog is that you don't allow him to interact with too many people. Only you and a couple of others should handle him. Over time this will ensure that he mistrusts any stranger. Don't let him socialise with too many people and he will automatically become wary of them.