It's a good introductory to Sageuk. It's like a pretty picture book come to life. Gorgeous costumes, charming characters and a fairy tale like love story. In as much as I enjoyed this, I couldn't give it the rating (9.0 and above) that I wanted to. I gave it an 8.5 purely because I enjoyed it so much but in reality, this is probably at 8 at best. This type of historical isn't for everyone and one's enjoyment will really depend on his/her expectations. To put it bluntly, if you don't mind watching a traditional love story (think of it like reading a Grimm fairy tale, not one of the revised fairy tales you see on tv) then you probably won't mind how male centric this is. The female characters are very likeable but you won't see any badass female leads here, save for the FL's bodyguard who doesn't have much scenes. Personally, I am the type of viewer who can also enjoy a traditional female lead as well as a tough one...so, while this was a source of disappointment for me it was not a deal breaker.

The reasons why I couldn't give this a higher score were:

The FL lead's casting: I don't want to criticize the actress but I think she was miscast. When I compared the child actress to the adult one, it really seemed as if they were two different people and not in a good way. This drama has 2 sets of characters. The child actors who take up the first 5 episodes, then the adult cast...and the FL's actress is the only one who seems nothing like the child actress :/ The actress playing the princess Minhwa, the FL antagonist Bo Kyung and even the FL's sidekicks (heck, even all the male leads!)...while they didn't exactly resemble the outward appearance of their child counterparts...they were still able to appear as the same person. Not so for the FL. And, I hate to say it, but she wasn't as good an actress as the rest. I compared all crying/high tension/emoting scenes of all the characters and only the FL's scenes appeared very forced and unnatural :(

The magical premise/the Shaman: I found the Shaman's character extremely annoying. It really seemed as if- in trying to 'prevent anything bad from happening to the FL', she basically killed her and robbed her of her life.
I get that there is a magical superstition involved here but the show didn't execute this very well so her reasons weren't convincing for me. She had one good Shamanic fight with another shaman at the end of the show and this is the only scene where I enjoyed her character. For the rest of her scenes I found myself rolling my eyes at her "I won't let anything bad happen to you so I will kill you and deprive you of your family" arguments. Everytime one of the other characters questioned her logic, she would resort to riddles and start blabbing about the moon and the sun! >< The sad thing is that she could have been a really cool character if they only executed the magic/superstitious plot points better. The shamanic element was also one of the aspects that made this Sageuk different from, say, Princess Man or Empress Ki. Too bad the most powerful Shaman was a moron.

I also wish the side characters (both male and female) were better developed and had different endings . Then again, it might not be fair of me to expect this, given that this drama only had 20 episodes and other plot points to deal with. Aside from the FL's Cinderellesque story arc, there was also the corruption in the palace arc, Prince Yangmyeung's existential musings., the rivalry between the brothers, etc. I feel that Chinese dramas like Eternal Love do a better job of developing side characters because they have more episodes to work with, not to mention a bigger character scope. But I gave this a high rating for a couple of reasons:

The main couple: I really enjoyed the main pairing, despite wishing a different actress was cast. They had really cute moments and good chemistry. It was mostly because of the ML's performance and it's a pity a better actress was not used.

The ML: Kim So Hyun was really* good here. I was surprised because he didn't make any impression on me when I saw him in You Who Came from the Stars (that drama was more female centric, and it was all about the fabulous Jun Jihyun); I guess in here it was the opposite. METS was mostly ML centered and KSH pretty much carried the show on his back. He was perfect in the role of a young king who could be so many things- authoritative, angry, bitter, heartbroken... and at the same time charming, boyish and devoted. Very different from his flat character in YWCFTS!

His scenes with the FL were some of the best OTP moments I've seen in romantic kdrama. Aside from that, I really loved the scenes where he was being sarcastic, even when smiling. I think his verbal sparring with his corrupt court officials were some of my favorites. His character appears passive first but develops better in the latter part of the drama. It might appear slow to some, but the 'adult years' actually open around only 3 years into his reign when he is still learning the ropes and how badly the court was run before. He turns out to a be a more effective, capable ruler than his father, who let himself be lead around the nose by others. This is a pretty intelligent ML here.

The second ML: Jung Il Woo as YangMyeung. He was pretty good as well in playing a carefree outsider. He managed to be both likable and yet very distinct from his royal brother. While I don't care for love triangles, his infatuation with the FL wasn't annoying whatsoever. It's mostly because the writer treated both MLs with respect. Certainly much better compared to the annoying catfights you see in Chinese drama where the women are unbelievably catty towards each other; unfortunately, even when it comes to love triangles, male characters get better treatment :(

The side characters: Despite the fact that the side characters didn't have as much scenes, their performances were all good, considering what they were given and the short time frame where so many characters had to be juggled.

So yeah, overall, there's a lot to enjoy in this, if one watches it with these things in mind. It's not as grand an epic, nor as deep as something like Eternal Love but I think this is one of the better historical kdramas.