I initially knew about AOS since long time, but it never interests me because the actors and actresses were mainly rookies except for JSM, but then the second season came out and then I was like, "If there is a second season then that means the first season is good right?" so I gave it a go. IT WAS VERY GOOD.

I am a character driven type of a person more than plot driven ones, and this drama served it to me in gold platter. Naksu opened the character with her intense battle and mysterious aura, paving the way into the drama's center plot. We are curious about this person. Mudeok is another interesting character, because it's supposed to be a 'fake' character. JSM played Mudeok very well, her expressions are so funny! It's just sometimes I see her playful kiss character flashing in Mudeok (?) so it kinda bothers me in a way, but I think it's not that prominent so it's fine.
LJW's Jang Uk is literally perfect. Jang Uk is the driving force in this drama, I think. His characterization is fleshed out perfectly and consistently throughout the season. Spoiled, repressed, lonely, innocent and deeply loving, it's hard not to empathize with him.
The supporting characters are all so good and very strongly fleshed out, no one is a stone faced NPC, everyone took part in the big picture of the drama.

Cinematography, CGI, coloring, wardrobe, soundtracks, the drama lines are all exceptional. You may find plot holes, but some will be solved in Season 2. Overall, it was very fun and entertaining and emotionally rollercoastered me into the finale, and built a strong foundation for a next season.

I would highly recommend this if you like a historical fantasy genre, or if you like angst with comedy, in general.