I agree with what most others are saying.
The ML was the best character and I found myself excited whenever he showed up on screen. It's hard to put this in words, but he is the most "king" character I have ever seen. He felt truly terrifying at times when people got on his bad side, but truly kind at heart. This dynamic made him really compelling. It's also one of the few times I've seen the power dynamic in these historical dramas to really play out. Most of the time, I felt like the FL was far too afraid of him. He gave her good reason to be, but I never felt like we really got over that well enough. At least on her part. I was fully "captivated" by him way sooner than she was, but I'm not the FL.

All in all, if you like political games, this drama has plenty of it to keep you interested, but if you want romance, there is very little of it. The king waists no time in letting the audience know how much he cares for her, and we feel it whenever he looks in her direction, or even just at the sky, but she seems really passe about the whole thing. She didn't even want to tell him that she returned from Qing and was going to leave quietly?! WOULDN'T YOU WANT TO SEE THE MAN YOU LOVE?! That was pretty disappointing.

The rest of the characters are fine. I have a soft spot for Lee Shin Young (Myungha) and Yang Kyung Won (Yoo Hyunbo), as they are my boys from Crash Landing on You, but especially Yang Kyung Won. I will watch him in anything, he's a great actor.