This is my first time giving a review, so it could be quite messy and unnecessarily lengthy. I'm halfway in, but I believe my thoughts are likely to stay the same until the show ends. I'll elaborate my rating for each category as below:

Story: 7.5/10
You probably read the synopsis in advance. Now back to my review.
The plot has a fine setup and the mystery factor tie into. Definitely not a simple one, but you can go along with it. You don't have to squeeze until the very last drop of your brain to connect the dots. The clues are quite in your face so you can guess some secrets right beforehand, even if it's a few seconds to an actual reveal on screen. But still, there are rooms to speculate and sometimes things turned out completely different from what some had theorized. Overall, it is unexpected and complex, but it's not crazy whatsoever.
About the romance factor, the relationships among the characters play out well, you can see the dynamics work. The love lines are still in the developing process as of ep. 10, but the love triangles do be a bit exhausting at times (yes, there are more than one.)
The script itself sometimes makes the execution fall flat for many details in the show. I can't specifically say which, but there are occasions when I felt some ideas can be explored further or given with a better explanation. Some scenes such as Park Jin and Miss Kim's bickering were extended longer than I preferred.

Acting: 9/10
To be frank, the acting is quite solid and convincing. Many of the cast are experienced and it's believable for them to pull off a good act. Even the so-called "idol actors" such as Minhyun and Arin both have the potentials and are capable to go further if they continue to improve themselves. The chemistry between the two leads, Jaewook and Somin is very over the place, which makes the dynamics of their relationship more real, hence why I ship their characters hard. Everyone has the visual and is talented as well. Perfect cast.

Music: 7.5/10
There is nothing much to say here, the tracks are played here and there, they definitely push the emotions, but they are not outstanding and leave no impressions to me. I personally would not look them up and listen to them.

Rewatch value: 5/10
Maybe I just rate this in a 50/50 way because I'm neutral about this. I'm not someone who rewatch shows, so I think I can't speak for the rest.

So that wraps up my overall thoughts about the series. You should give it a watch if you are interested in the fantasy genre and want to have a show as an escapism. It's a weekly show as well, so you can take breaks between the episodes to let it sink in. Hope you'll have a good watch!