For the first 10 episodes or so, I was a little lukewarm toward the series, mainly because I found the pacing to be a bit slow, and I wasn't on board with the main ship yet. However, my decision to persist was the best decision yet. I grew to love not only Gu Dong Mae (I'm obsessed!), but also Kim Hui-Seong, Eugene Choi, Lady Ae-Sin, Kudo Hina, and all those side characters as well. This drama is so heavy, and at times so light and uplifting. It has left me with so many feelings with my heart. I'm overwhelmed and utterly impressed by this experience.
Others have said so much about this drama, but I just have to say this:

Humor is 10/10, and this is coming from a person who is very skeptical of humor in k-dramas. But this was so well-done, everything from Interpreter Im and Do-Mi, to the bromance between Eugene, Dong Mae and Hui Seong, to Ae-Sin's struggle to learn English. I'm left wanting a special episode for each of the main characters that I wanted to learn more about; namely Gu Dong Mae and Kudo Hina. I really, really wanted to learn how Kudo Hina came to be who she is when the drama begins. If you're looking for complex characters, this is the series to watch.

Acting 10/10: All characters, main or secondary, completely owned their roles and became them.
Cinematography 10/10 ... need I say more? Every shot was breathtaking.
Plot 10/10 I freaking love historical narratives.
Romance 9/10 I didn't buy Hui Seong's love for Ae Sin (that was too abrupt and I don't believe in love at first sight), but the Ae Sin - Dong Mae interactions were full of tension, and the Ae Sin Eugene interactions always made me smile. It was adorable and heartbreaking.
Pacing 8/10 If you're like me, you'll get impatient with the drawn out scenes where a character just stares into the distant with a pretty background and a melodic OST blaring in all its glory. I just zoomed past those, but there were too many of such scenes and I hungered for more action ... which I got after episode 12 (so hang on!).

I really liked how the drama portrays class conflict between characters entangled with political/military conflict and nationality. Eugene Choi is a Korean-born American captain who seems to struggle with how he should approach his feelings toward Joseon, a country that failed to protect him and his family. Ae-Sin is a noblewoman who is deeply loyal to her country, but is stuck between the patriarchal restraints on women, the need to pick up a weapon and fight, and a complex relationship with lower class like Gu Dong Mae. "The hierarchical system is destroyed, but we can't change our roots. Once a slave, always a slave." The series doesn't merely portray the unfailing courage of Joseon in the face of Japanese imperialism, it also highlights some key problems of the Joseon society. In Joseon there are the lower class butchers and slaves who are so oppressed they can't even protect their own children, there are traitors who are keen to sell Joseon to Japan, there are those who turn against their own parents and choose to be loyal to their country, and there are those who leave their country to survive, only to come back and take revenge against those who oppressed them.

This drama truly deserves the high ratings and accolades it's gotten.