My rating is greatly influenced by the romance in this drama, because of the quality of the production and the moments that left you on a high, it was enough to make me eager for Fridays. If you are looking for a well thought out sci-fi/fantasy drama, this is not for you. But if you can abandon logical storyline and live in this world of make believe for a king and his girl, you are good to go.
I was reminded of Dr. Who when I watched this but the world created is nowhere as exquisite as in Dr. Who. The accepted lore surrounding time travel and space/time stories don't bother this story, it does its own thing. Sounds like you should not watch ---- but as much as all of this is true, I felt that this show delivered a romance that made it worth a few hours of my time.
Story:
I gave the story a 9 because of its bones. It is not fleshed out well, but there is enough for someone to pick up and let your imagination take you the rest of the way.
The high production value does enough to keep you in the world of the drama even when the story lines when off the rails sometimes.
There is a theme of love across almost impossible boundaries like that which can be found in Crash Landing On You.
I don't mind cheesy romances... I can discount the plot holes and live for the moments where the leading man is dressed in his military dress uniform and stand stately and claim his woman.
Acting/Cast:
Lets just say Woo Do Hwan 10/10 ….
I don't favour Lee Minho's acting but he suited this character and did a credible job of creating the mood of the romance. A cool, smitten, dominant male lead. It isn't a swoony romance and it is not about the female lead. Lee Minho carried the romance for me but it might be because I like a strong male lead. This role seems to be suited for his type of acting. The charisma of a King or a knight in shining armour on a white horse. There were enough times he got to say lines that cheesy romance novels stereo typically write for a King.
The rest of the cast, considering everyone had to play dual roles, I found extremely engaging. The storyline failed a couple of the characters in later episodes.
Music:
10/10 and really, all I need to say is that there is this song: https://www.youtube.com/watch?v=apNWEcuBboI
Rewatch:
I don’t think I would rewatch this without a lot of fast forwarding.

Overall:
You can break a piece of art down into the different lines and colours but until you stand back and look at it as a whole, you don’t really see any substance or merit. This show is like that to me. There is so much I can say that could have been done better. Yet, I feel I could replay some of the moments and get the “feels” all over again. Watching it left me feeling good. If that is all it provides in the world as it is today, then this drama was worth it.