Sadie - The old castle represents your defences against being hurt or attack. Often they are built when we are young and someone says something hurtful or a parent says something negative. We may react by feeling angry, retreating, wanting to hit back - and they are simply some ways we create defenses such as a castle. I came across one in which I realised I had steel doors which I closed when I felt someone didn't understand me or judged me. I closed any feeling of warmth then. So when I felt myself closing the doors I would say, "I am closing the doors, and it would help stay open."
But your dream shows you are progressing in the fantasy land - your inner world. See
http://dreamhawk.com/inner-life/inner-world/The tasks you met and the dragon are all symbols of your own wonderful potential, and represent stages of personal growth. When you were a child the process of Life pushed you on to meet enormous challenges. You met adolescence and sexual awakening, and how did you deal with it. You killed the dragon of instinctive sexuality - an old Christian attitude because sex was seen as a sin except in tightly controlled situations. But other cultures related to it differently, and learned to tame and fly with the dragon.
The three tasks are deeply planted in the instinctive and often unconscious part of us, the part which keeps you breathing and moving while your personality you know as Sadie is lost in sleep are sometimes bring slight awareness of its treasure of knowledge in dreams.
The ginormous dog is another aspect of your instinctive wisdom that had been left by his owner - you are the owner of your own inner world - for thousands of years. This may represent that you as a person are the result of thousands of year of culture and its language - none of which you created but yet are the result of - thousands of years of Christian and scientific culture, which spent a lot of time denying our instinctive unconscious.
Well, you did a great job in resurrecting the living dog. See
http://dreamhawk.com/dream-dictionary/dog/The tasks were defined thousands of years ago, and represent what we face or need to learn in becoming a full woman or man. In Africa a tribe realised that young men and women were actually instinctive animals until the faced a trial which made them human. In today's world most of us are still instinctive animals, eating too much, developing enormous teeth to kill with - the fascination with guns in some nations and groups; the belief that those of a different race or belief are to be killed - an animal trait; the enormous belief that sex is the fullest expression of living - animals are consumed by mating and fighting for a mate; the enormous desire to dress and look like every other animal in our group - animals do this as a protection.
So you in your dream are on the way to becoming a real human. And one of the first tasks is finding out what our potential is. "What are all of the seeds of possibility in your psyche of your world? How much money do you have in the bank? How much energy do you have for this? How much talent do you have for this? What are you putting together out of all your possibilities?"
The second task is to find a balance between being eaten and eating, between competition and being a follower.
The third task, according to the legend of Psyche (the soul/personality), was to gather the waters of life and give it form by living from awareness of Life in you. This means expanding your awareness until like an eagle you can see your whole life and the world around you as from a great height.
In the myth there is a fourth step which needs you to enter the unconscious and face the actual death of your present self. Search for The Myth of Psyche
My advice is to keep following your dreams and try to understand them by using
http://dreamhawk.com/dream-dictionary/practical-techniques-for-understanding-your-dreams/Tony
