I am usually on the fence when it comes to Sageuk or Historical Korean Dramas - they tend to either be very well done or they come off as cheap looking and over-dramatic. Thankfully, this show is the former.
Stunning cinematography, an interesting premise, solid writing, and fantastic acting make this an all rounder. The casting director should get a raise for casting Jang Dong-Yoon as Nokdu - the man dressing as a woman bit would never have worked if not for Dong-Yoon's unique look which enabled him to look really beautiful and convincing as a woman but equally as handsome as a man. Chemistry between the leads is great and this show has some really genuine romantic moments that had me swooning. All the characters are really well developed, from the side characters to the villains, making character actions and motivations even more convincing. The soundtrack was wonderful, never overplayed and really gorgeous at times.
Also I just have to say I loved that Nokdu actually had long hair for this role - its a small detail but it made a difference, as the hair buns on other actors are always so fake looking with the short hair in the back.
This show felt similar to "100 Days My Prince" except that this one was SO MUCH BETTER. This show delivered on both romance and a satisfying ending, unlike 100 Days which I am still so mad about!
Definitely give this show a watch, it has great action, romance, and wonderful acting!