This drama is just very very good. Here the reason:
-Production: I would the production is very high for this drama, each set is very well design to fit in their story from the palace to the mage school. Since the drama has its own set and time, so the design is its own too. The costume is also very beautiful too.
-Story: all the character has its own side story not just ML. And the way they build up each character is just very satisfied. ML has very well story while didn't out shine the other SL.
-Cast and Performance: this alone is already the best, they perform each set so perfect. I would say well experience cast in this drama.