I was gripped from ep1 and found this drama a cut above the rest of the more like-real-life based historical dramas.
The characters are wonderfully diverse, considering they are almost all Royalty, politicians or servants.
Of all the many dramas of this type I've watched, not many actually make me contemplate the polar acts of revenge and acceptance, or of putting the desire for a peaceful, unified future first, ahead of personal feelings of hate and injustice.
This was, at times, very violent and hard to watch. Not because it was particularly gory, but more because of the way people were used as pawns; ripped from peaceful lives to be tortured and villified, with entire families wiped out, purely to suit a desired outcome.

The same reason that different characters have, to murder and plot, are revealed through each episode. It is a vicious and cruel circle.
Yet only 1 character seems to identify this and it changes the course of their own path, which underpins the story.
There is plenty of action, not all of the sword fighting type either. It's a well thought out and intelligently written story.

I did feel that as the series neared its end, however, things got rushed in comparison to the early to mid episodes. If limited to 16, it's understandable, as apart from hugely pregnant pauses between characters at times, I couldn't think of anywhere that earlier scenes/eps could be shortened.

Jang Hyuk is so good at portraying the type of character that he does in this drama. There were odd occasions when I felt his signature booming voice, when raised in anger, was used inappropriately, which made it lose its impact. That, however, is not down to his acting, but to direction.
(I honestly feel actors are too often blamed for the poor choices and decisions of Directors in how they want the MCs portrayed. A great actor will struggle in a badly directed drama or film; the Director, not the Star, is the boss on a set.)
He is very good at adding weight to a character and at times one can almost feel how the character does, through him, and Park Gye-won is really something. Traumatic and horrifying experiences, a total loss of trust in any King's ability to rule without indiscriminately spilling blood and certainty that his resulting actions are for the greater good, leads to yet another tyrant being in control. What he was willing to sacrifice (and force others to) and do, to have his perfect Joseon protected, was cruel, selfish and heartless.

Lee Joon is very good at playing characters who veer from sensitive and caring, to deeply cold and frightening, in a split second. His malevolent stares are second to none (with Jang Hyuk's matching on several occasions). His tragically steep learning curve from the idealistic, naive young Crown Prince, to becoming a King that's forced to live in constant fear for his life, controlled and manipulated, again produces a character who trusts no one and believes his means to attaining revenge and stability, are justified. Due to the aforementioned rushed final episodes, I did feel his character's personality suffered a bit of whiplash, but his reaction to the loss of a significant character kind of brought it back in check. There was the odd occasion, however, where I wondered what on earth was going on with him! Again, that's no reflection on Lee Joon's excellent acting, but on the writing, pace of the story, and its direction.

Kang Han-na was well cast as the FL; Yoo Jung's lot was really hard and yet she presents with such dignity, patience, strength and foresight. This character, to be honest, felt too good to be true and would give Mother Teresa a run for her money. Yet, I still bought into her and admired what I felt the story was getting at, in her choices and decisions. Revenge has to stop somewhere and irl, history is full of examples of the bloodshed and lost lives caused by living in the past and seeking revenge on the future generations, of those who committed the crime, regardless of whether they were involved, or pose the same threat.

Of the other characters, I thought the Queen Dowager (Park Ji-yeon) was another example of personal hatred, jealousy and fear taking a person over. The surface is a lot more beautiful than what lies beneath, although she was used by a character for whom her love never really died, but consumed her. Another, Gye-won's wife, Lady Yoon (Seo Yoo Jun) was the epitome of loving someone more and putting her duties as supportive wife, mother and family protector, before all else. I wonder how he would have been, if she'd been more like Yoo Jung.

The relationships between and types of other characters, were pretty well done, even if not as in-depth, I got a sense of the dynamic, affection and type of people they were.

As mentioned, the ending felt somewhat weak in comparison to the rest of the drama (if it hadn't been so good, maybe I wouldn't think that), but at the same time, it didn't ruin it for me. I'd just rather have had more. Some things were over simplified to fit, I thought, and what becomes of several characters, after everything that happened, was what stopped me giving this a 10/10. More thought should have gone into it.

Jo Yeon-hee (Minister of War's daughter and Royal Concubine also in running to be Queen), was vile; she hadn't had a hard life, but was totally spoilt, with no regard for people below her ~ she saw them as chattle and their lives were hers to take if needed. She felt no remorse, no guilt, nothing at all ~ only that she might be in danger herself as a result and be hated by the King, whom she adored and coveted. Her actions, schemes and desires should never have gone unpunished; I was livid!
The end marked a beginning and I interpreted it as alluding to the fact that their future would not be easy. Their decisions made, to not cause more bloodshed, would likely create danger and plotting against them in the future, but they would deal with them as and when; and not take the easier path of their forebears, murdering, creating cover ups and scape goats to cover their tracks, 'just in case'.
This drama is really well worth watching; a classic.