The story, especially the mystery aspect and how the protagonists figure it out and set things right, is compelling. The acting is remarkable, particularly those of the lead characters, Park Hyung Sik as the crown prince and Jeon So Nee as Min Jae Yi / Go Soon Dol.
Though it features gender-switching, a trope I'm not too crazy about, I don't mind it that much here as the characters who matter know about the subterfuge early on. And when you're wrongfully accused of murder, what could be a better way to hide while you try to find the real culprit?
The dedication of the crown prince to do what is right for his people is awe-inspiring. That scene where he is struggling between duty and love, between marrying the chosen crown princess or following his heart is so powerful I cried, though it is depicted so subtly with no tears shed, just his face showing his heartbreak. I also love how he chooses peace and forgiveness at the end instead of going all out shedding more blood to avenge every single wrong committed by the enemies.
The ending is lovely. I wish all dramas have endings as heartwarming as this.