It's an amazing Korean cinema as always. Had to wait a long time to watch it as it's been out for long. The lead actor is amazing as he always is and the veteran actor playing the King is always a screen presence. (No need to write out names since this site is mostly western users).
The story and the cinematography stands out, the acting and period piece settings makes a statement as should, but I won't deny this wasn't the way a movie like this should end. It felt like the twist and turns that made it work for 70% of the movie became so exciting for the writers they decided to push it further unnecessarily. A few major character arcs were left out very dully (kings wife, Prince wife, Prince son, etc). The other twist with a major actor seemed probable but somewhat unsavory, and didn't even have a final pay off. And in the end, a few things felt like, "let's just end it this way."
Maybe if the plot with the world becoming modern was explored a little bit, some things could have been more relatable, but it wasn't. Even the King's back story could have helped if there was a better explanation or flash back. (A part of me feels this was intended to be a 6 part mini series but budget wasn't there).
Regardless, for what it intended to be and was for a major part, it was amazing. Wish it ended better.