DO NOT BELIEVE THE NEGATIVE REVIEWS. This is one of the best historica dramas out there. The only thing I agree with: the poster is misleading but only a little, because at the heart, it's a socio-political drama. This shows highlights:
The foundation of trust and friendship is what matters most;
that agency of women has been hard fought and we are still fighting;
people in power will do anything to keep their power unchecked;
social injustice unhinges people; revenge is never pretty and is a vicious cycle, and one day someone has to sacrifice something to break it.
learning to be humble and thankful for things we often take for granted (freedom to make choices, having enough to eat, having a roof over the head, trusting people around you when anyone could kill you anytime);
And it's about making the right choice. This drama also features one the most FEMINIST heroes that I have seen in a very very long time. It's unbeleivable how beautifully that arc has been shown. His belief in his woman's capabilities is limitless and is a pure delight to watch. He is never the knight in the shinning armor, and only steps in if his woman is facing trouble because of others' prejudices. As Jay Yi beautifully says in one instance "he is a man who breaks down high walls for a woman who is stuck behind it'.

It IS a romance but not the regular kind we see today. It's more a vehicle that moves the narrative forward. Yi Hwan (the Crown Prince) falls in love with a woman he has no chance with - she technically belongs to his best friend (whether she wants it or not, she is just an object in the eyes of the soiety she lives in), while the best friend has this unhealthy attachment to a woman he has never met as an adult, making life truly difficult for the Prince; plus she does not fit any criteria to become his Crown Princess even if she were to be absolved of the murders (The Crown Princes were governed by strict rules of marriage, he had no choice in who he could marry as it was always a politica decision taken by the royal parents; the Kings however were not bound by any of it, they could choose their own Queens if they wanted). Min Jay Yi, duty bound to marry the man her family chose for her, marriage being a contract between families with no place for choice/love, is now an accused murderer on the run and, not only she loses everything - her family, her home, her reputation, her identity, but she also ends up falling in love with the most unatainable man she could imagine.

Hence, they have no chance with each other. But when does heart ever listen to reason? Yes, they never touch each other and totally adhere to the Josean principles of physical space between a man and a woman at all times. But their relationship is so wholesome, beautiful and pure that it harks back to the times when romance was all about yearning and longing. Don't miss their interactions, they confess to each other throughout without saying the actual words and it's beautiful to behold. And their interactions are hilarious at times. He is stuffy, morally upright, naive and insanely competitive. She is fiesty, unconventional, motor mouth but keeps her innermost feelings bottled up because that's how she has been raised. He falls for Jay Yi for the world she shows him outside of his very lonely existence (she is fearless and she drinks haha!), someone he can trust, rely on, beleive in, and laugh with, while she falls for his progressive thoughts, and benevolence. Both are brilliant people and never fail to complement each other for that. Or take each other down a peg when the situation demands.
All of these are packaged into a constantly moving narrative, packed with super funny moments and sparkling chemistry between Park Hyung Sik and Jeon So Nee. The side characters are to die for - Myung Jin and Ga Ram (Lee Tae Sun and Pyo Ye-Jin) are fabulous and the four make an investigative team (Shield team) that eventually resolve the mystery, with each of them bringing something to the table. These four could easily have a spin off show. Jeong Da-Eun as princess is super adorable and a very interesting character. The main villain is one dimensional but at the end, that makes his end supremely satisfying to watch. But it's the backstory that keeps you hooked.

The cast is steller, but Park Hyung Sik is ethereal as the Crown Prince. Not only he is blindingly good looking, but boy, can this man act. He brings Yi Hwan to life, and he does it with just his voice and his eyes. You cry with him, you laugh with him, you feel his heartache, you feel his jealousy, you feel his helplessness and rage at the injustices he is trying to fight . This is an award winning performance and hope he gets recognized for it. Jeon So Nee is a perfect foil to him. She starts ordinary and then grows on you. The fact that she is drop dead gorgeous is a bonus (take her out of the Eunuch uniform, put her hair down, put her in a plain unfashionable white gown and she takes your breath away). She goes through various stages of her performance with aplomb, and absolutely aces it in the emotional scenes (her confession will give you goosbumps and stay with you for a long time). She is an actress to watch out for.