You can tell straight away that a lot of work has been put into it: from the costume designs (always super detailed and in accordance with the historical theme), to the character development. (SKIP TO END IF YOU WANT BRIEF CONCLUSION)

First, I think that, compared to the art in other korean manwhas, this one's was pretty good. I had a few moments when I thought "Omg, that must have taken ages to draw, it's really detailed and beautiful". The clothes and buildings are really nice and varied.
Second, the story. This is going to include some venting though... The idea of the story is GREAT. If it were written slightly differently, it would be the PERFECT shoujo that I would read again and again. BUT: the way the author wrote it, it soon became very tiring and long. It was very repetitive, obvious what was going to happen, even when there was an event that surprised me, I could predict who and how was going to make it go wrong. In the end, you just want to give up because of how the main characters are constantly torn apart again, and again, and again. It kind of feels like the author just spread the story as long as possible to keep it going longer, which ruined it. If it had been a few volumes shorter, and had a better ending (you know a better "reward" for reading all of that), again, it would have been rated one of my favourite mangas.
Finally, the characters. This was one huge flaw: there were a lot of times where I was super confused about how they had come about a decision, or how they fit into the story. There is also a huge focus on background characters, which only confuses you more, (eg the eunuch and Yul's relationship is super bizarre from love to hate to something awkward...). The main characters and the king/queen are the only consistent characters. They are really well developed and aren't "black and white" like most mangas would make them: they change, don't do what you expect them too, and act like real human beings would (I think this is the charm of the book).

So all in all, this is a good read, I'm glad I read it, but I seriously think that once you get to volume 12 or 13, it all starts going downhill and repetitive. I was tearing my hair out by volume 17. I think something the author overlooked is that when someone picks up a shoujo, it's obviously because they want to feel good about a relationship, like catharsis: overcoming the obstacles with the couple and then being rewarded and happy when they can be together. This is absolutely NOT that. If you want a nice romance that you can stupidly smile at (unfortunately a habit of mine ;) ) and cry and finish with a huge grin and feel giddy, DO NOT READ THIS. But if you want a proper story, basically a novel with pictures, then this book is defo for you, and is very well written (malgré all my bad-mouthing... I was just so frustrattteeeed!!!).