This show is probably my favorite K-drama yet.

Positives:
- Song Kang's acting is amazing (and he's just very gorgeous)
- interesting and well-written female lead
- well-thought out story line and fantasy elements
- funny side characters
- interesting villain
- thriller/gore-like elements
- very strong and good start & middle
- the soundtrack (the instrumentals!!!) is so good
- the insight into history and religion were interesting

Negatives:
- the last two episodes are much worse than the rest of the series and can't really compare - the plot and the character development feels out of place and uncharacteristic. This would have been better without the last two episodes (whose conflict could've been solved within minutes)
- some characters are a bit ridiculous/unserious and there are a few cringy moments
- the villains were a bit predictable but that wasn't the point, so it's fine

Overall really a great series. I especially loved Guwon, he was funny, pretty, epic, entertaining, and just great in general. The only real downside is that the show drops a bit in quality after/around ep. 13, but that is the case in many K-dramas.