The traditional archetypal hero is usually the protagonist, usually male, has an ultimate objective and overcomes obstacles along the way to achieve his goal. The hero is morally good and his goodness will be usually challenged along with his story.
The reason Arthur can be viewed as a traditional hero is that he is the chosen one to remove Excalibur from the stone. Merlin the magician made magic so only the “rightful king” would remove the sword and be the ruler of Britain.
After becoming king Arthur defends Britain against the Saxon invaders - it can be considered one of the obstacles he has to face. He was a fair man and his knights would seat on a roundtable that meant to show that all of them were equal - this shows his morality and goodness.
