I thoroughly enjoyed this drama! The characters, music, and plot are all stellar. Be warned that the first half is a bit heavy on the lore, worldbuilding, and background--I personally did not mind this, since it set up the second half well, but if you're mostly looking for action, you'll likely enjoy the second half much more.
Having not heard of any Korean dramas in the ancient genre before this, I was intrigued by the premise, and I was certainly not disappointed after watching the series! By the end, there is no clear-cut villain, and it is impossible to decide who to root for. Song Joongki did a great job with the dual roles; if someone had told me that his two characters were played by two different (twin) actors, I would certainly have believed it! Eunseom starts out as a sweet guy trying to save his friends and ends up inciting a rebellion with armies at his beck and call, which I found to be really cool. Saya is darker, with a harrowing past. (He is also very pretty.) Tagon and Taealha were great villains, and although they were the bad guys, I often found myself rooting for them. I liked Tanya a lot too--she is a very strong character and always willing to do what is right. I highly appreciated that there was only a very miniscule amount of romance; nothing fluffy, cringey, or time-wasting.
I have to give a shoutout to the side characters too. I loved Dalsae, Yangcha, and Karika. And of course, I was very impressed by Kim Sungcheol as usual--his versatility as an actor is amazing, and Ipsaeng quickly became my favorite character. He was a lying, stealing, backstabbing trickster who was a lot of fun, and I admittedly enjoy (maybe a bit too much) seeing Sungcheol and Song Joongki acting together. (Vincenzo episode 8, anyone?)
If you're looking for something fresh, with bloody swordfights, political discourse, and fantastical elements, this is the drama for you. I reiterate that if you're thirsty for action, you might find the first half to be a bit slow, but the second half will definitely be worth it. The story touches on many real-life historical issues, such as colonization and slavery. I have seen this drama often compared to Lord of the Rings and Game of Thrones, but the only similarities I saw were the type of setting and the aesthetic. The show is very unique (especially among Korean dramas) and I highly recommend it! I'm anxiously waiting for the second season and can't wait for it to come out!