The cast, the OSTs and the plot was a lot amazing. Special mention to Kim Kwon for playing Seongmo - I cannot imagine anyone else playing him. He nailed his character. Jinyoung did a really good job too for his first main role. I've seen his other dramas and I'm proud of what he has achieved and his improvement.
It has the balance of romance and suspense that you're looking for.
You have to watch this drama if you want something that keeps you on your toes and something out of the ordinary and fascinating. There were even some scences that foreshadowed future events - I honestly though this was brilliant how the writer managed to do this.