My 4.5 year old Rotty/Alsatian has recently started to become very assertive. He is very territorial with our property and his possessions, balls, food etc to me his main carer and other family members, husband grand children. Is it a fase, an age thing? How do I challenge him. I am very assertive with him but he challenges me back.

Has anything changed in the household? Have you recently moved, adopted a new pet, etc. etc. It seems like he may be reacting to something in the home.

Is he fixed?

This is not a phase and needs to be addressed. Was the dog well trained earlier? Did he recognize all humans as his alpha? When you go up stairs who goes first? When you reach a doorway who passes through it first?

Is he neutered? Male dogs reaching sexual maturity do have an attitude change.

If he is neutered, then I would suggest to find a trainer to help you. Any advise given over the computer has a potential to be very dangerous. You need to find a trainer who specializes in aggressive behavior and how to manage it. Some dogs are just bullies... It'll never be trained out of him, you'll just learn how to handle him better.

I commend your bravery, but I have to fault your wisdom in challenging your dog.

You admittedly don't know how to handle this situation, but you are willing to escalate it into something you most undoubtedly will lose. At some point he may simply take you up on your challenge instead of returning it. So, stop challenging this dog. Get some professional help and save the experimentation for a much smaller dog.