Being only halfway thru the series, I almost waited until the end to write a review. This show is just too good to not have any reviews up though, so here's what I've seen thus far.
Many Korean dramas are based in the country's history, and this one is also. Not only did the writers pick a particularly tempestuous time period, but also father/son rulers. The story so far has grabbed me from the very fist show, and there's no way I would stop watching through the entire 50+ episodes. The story is written in such a way as to carefully craft each and every character interaction, to best acquaint the audience with all the players. Their stories are being quite intricately developed with many layers. I've found myself sad when some of the worst villains were done away with, because I enjoyed them so much. There are not so many black and white characters, as many shades of gray characters. The writers and extremely talented actors make each character so human, it's easy to forget this story is told in a different century!
The entire cast seem to be the best in the field, especially the young men and women. The Lee Bang Won character, played by Yoo An-in, has been absolutely astounding to watch. I've never seen such a young actor bring so much talent to a role. His scenes are some of the most compelling I've ever watched, in any movie or television show. All of the actors are superb in this series.
If you've ever been curious about shows from another part of the world, I highly recommend you watch this show. It's a work of art in every way.