No (alternative)

"I don't see why I have to kill you." you reply. This takes the dragon by surprise. "The way I see it, I only have to defeat you to prove my self worthy of the princess. You could say that by finding the back door I have defeated the trap you set for me and therefore defeated you. Since you no doubt want to live I do not need to kill you there is no reason to fight. Therefore, I propose a solution: I did my research and you have been bound by a magical contract is that right?" You ask.

"Yes" replied the dragon "I must not let anyone see the princess, other than myself and the king who must present me with gold in order to see the princess. I am to stay within the castle ground at all times while the princess is in the castle." The dragon explains. "It is a very restricting contract, I long to fly in the sky, roam the land, find a mate but alas I am stuck here and can only fly as high as the tallest battlements before the magic forces me back down. "

"Well then, I may have a solution for you. If I marry the princess I will be king and she will no longer be the princess thus ending the contract. Therefore, all you have to do is admit defeat to me and I will become worthy of marrying the princess and therefore as good as king. This means you could let me pass if I gave you a gold coin, which I have, and once I have passed I will be able to marry the princess and make her queen so that you are free to do what you wish. So, do you accept that by finding the back door I defeated you?" You ask.

"Yes, I accept defeat" replies the dragon, sounding happier than you expected probably because all its dreams were about to come true. You give the dragon a gold coin and move towards the tower that holds the princess.