at first, i was intrigued by this drama because of it's similarities to Goblin as well as eventually being pulled in because of it's strong casting. although i have to admit that the drama isn't perfect, it's still a good watch and i would strongly recommend it - the plot combines fantasy, romance, comedy, whilst not focusing too much on the political side of it which may bore some viewers.

story;
the plot of the drama isn't original, but it's a genre that keeps giving and giving and directors manage to put their own unique twists into them. in this instance, it's a parallel world comparing a modernised korea with a cultural monarchic korea being run by heart-throb Lee Min Ho ;) on the one hand, the plot is quite refreshing as it combines so many different elements of a typical kdrama and combines them into one: detective, historical, romance, fantasy etc... however something that puts me off is the initial weak chemistry between the two leads. it's presented as a one-sided love but quickly rushes into a romantic relationship with not much development other than a kiss pretty much.

acting/cast;
as said before, i was pulled in because of it's strong cast. this drama has stirred up a storm because of it's casting of legends like Lee Min Ho and Kim Go Eun. they both play pretty badass characters however they show their cheesy cute sides whenever it comes to developing their relationship with one another.
Lee Min Ho plays the role of a young attractive king very well, seeing as that's pretty much what he already is in real life anyway.
meanwhile Kim Go Eun plays a badass lieutenant in the police force who beats and chases down criminals. i'm somewhat in love with her character seeing as she's not generic in the way that she is vulnerable and allows her male counterpart to do all the work. she's sarcastic, funny, and charming in all the right places and i would definitely rewatch this show if it's just for her.

all in all i'd definitely suggest that you give this drama a go! it's refreshing with such a good cast that you don't want to miss out on :D