Disclaimer; You don't have to agree with what's written below, it's just my opinion.

"The Princess Royal" is set during the era of the Xia dynasty.The nobles were in power. The emperor hates the noble families. Hence, The princess Royal "Li Rong" couldn't marry from any of the noble families. "The Princess Royal" tells the story of Li Rong and Pei Wen Xuan who married for political reasons. Li Rong married Pei Wen Xuan because he's not from a noble family, Pei Wen Xuan too did so too because of his political ambition. As time pass by, these two eventually ends up standing on two different sides. Li Rong became the leader of the nobles, while Pei Wen Xuan became the leader of the commoners. Year after year, the misunderstandings of this married couple made them grew much further and further away from each other. Li Rong as The Regent Princess Royal had the best interest of the nobles at heart, while Pei Wen Xuan as the Prime Minister had the best interest of the commoners. After 20 years, the misunderstandings eventually resulted in the death of these two. Through a strange phenomenon, these two travelled back in time, 20 years back in time, exactly to the time when they weren't married. And thus, a chance to correct their mistakes. Will these two be able to correct their mistakes and clear up their misunderstandings?

Their past life misunderstandings explained;
In the past Li Rong thought Pei Wen Xuan loved another while he was married to her, that's their first misunderstanding. The second one was that Li Rong thought Pei Wen Xuan poisoned her in their struggle for power that's their second misunderstanding.
I love how the two couple cleared one of their misunderstanding. During the first four episodes, when Li Rong asked Pei Wen Xuan why he killed her, he told her, he wasn't the one who poisoned her. There she knew that the one who actually poisoned her was someone else and not Pei Wen Xuan. I love how one of their misunderstandings was cleared up.
The second misunderstanding about their feelings was cleared up during the first half of the drama.

In the past, only the nobles are able to hold high positions in the court, while the commoners only held low positions. This is one of the main reason that led to Pei Wen Xuan and Li Rong's death. In the present life, Li Rong and Pei Wen Xuan decided to get married for the second time and reform the laws of the imperial court, allowing the commoners to hold high positions like the nobles. But a reform is never easy, they'd have to face many oppositions from high rankings court officials. But that's why it's also a political drama. Li Rong and Pei Wen Xuan will have to overcome the many adversities that comes their way while reforming the laws of Xia. I love the main couple, although, they weren't the best couple I've seen when watching a drama, they had a unique chemistry. Yes, Li Rong was unsure about her feelings, and that went on for quite some time, but she later came to realize that whether it's the past or the present, the only one she truly loved is Pei Wen Xuan.

I love the interaction of the two couples. Theirs is one of the best interaction I've seen when watching a drama. I particularly love the strong character of both Pei Wen Xuan and Li Rong. Zhao Jin Mai delivered in her role as the strong, scheming and intelligent princess "Li Rong" excellently, Zhang Ling He also delivered in his role as the strong, intelligent, and scheming Pei Wen Xuan very perfectly. I love how well this two planned their schemes. I love strong FL and ML when watching a drama, and this drama has that.

IGNORE THE NEGATIVE COMMENTS
There were comments stating Li Rong was pining for Su Rong Qing, even after learning he was the one who killed her in the past. Ignore those comments. It's not what it seems. Whether it's the past or the present, Li Rong loved Pei Wen Xuan. Li Rong grew up being thought that affection is but a fleeting cloud in the face of power. Also, she had trust issues. She grew up seeing her father growing weary of her mother. Furthermore, the emperor who is also her father always guards against her and her brother who is the crown prince because of their mother who's also the empress being one of the noble families, and the most powerful one at that (the Shangguan family). So, she's weary of being hurt and betrayed. She tries to convince herself that her feelings for Pei Wen Xuan wasn't real, but she eventually give in to her feelings for him. I love how she came to realize that Pei Wen Xuan is the only one she could trust whether it's the past or in the present.

Some said Li Rong was angry when she found out that Li Chen was the one who killed her in the past, but she wasn't that angry when she thought it's Su Rong Qing did it. Sometimes, being stabbed by your closest kin is much more painful than being stab by a friend. Who's Li Chen to Li Rong? He's her closest kin, the main reason she ventured into the political world was to make sure he ascends the throne without a hitch. But then she finds out he was responsible for her death? Who wouldn't be angry? In my opinion, it's a natural reaction.

There's also complaints about Su Rong Qing's screentime overshadowing Pei Wen Xuan's. But I'm telling you this, "it's a false alarm." During the first half of the drama, Su Rong Qing was almost irrelevant in the drama. He only started getting a noticable amount of screentime during the last ten episodes which I didn't have complaints about. After all, this is a political drama, and it's only natural for the villain to get some screentime which Su Rong Qing happens to be. Pei Wen Xuan has more screentime than Su Rong Qing, that's a fact.

I love how the schemes and how the politics was portrayed in this drama. The schemes were well executed. I particularly love our main couple, PeiRong. Zhang Ling He and Zhao Jin Mai chemistry is both unique and natural in this drama. Not to mention the beautiful OST. This drama delivered beautifully. As for me, I didn't have problems with Li Rong, and the negative comments didn't sway me because I only focused on the main couple "PeiRong".

Ending Explained.
I've watched quite a number of dramas which had sad endings, but this one is an exception. After much scheming, the final battle finally ensues. The battle for power, the one who climbs to the highest position possible. Pei Wen Xuan and Su Rong Qing had a final show down after their long standing grudge. Their swords clash, and Pei Wen Xuan won. Su Rong Qing burned himself to death while playing the Zhiter. Li Chen ascended the throne. Pei Wen Xuan and Li Rong finally made a reform and eliminated the corruption that has been the norms of the imperial court, now, the nobles and the commoners are able to be high ranking officials. In other words, the aspiration of Li Rong and Pei Wen Xuan of making the commoners and nobles equals finally was realized. Pei Wen Xuan became the highest ranked official once again, and LR became the Regent Princess Royal again, but they accomplished all these things side by side, hand in hand. In the past they achieved great things while standing on different sides, which eventually led to their doom, but in their present life, they accomplished far more greater things together, as loving couples.

I love how the villains were punished. I loved more how every character got to be with their beloved, in which all those couldn't be achieved in the past life. A special episode was released. There we see the two children's of Pei Wen Xuan and Li Rong, and their married life as couples. I don't know about others, but I enjoyed "The Princess Royal", and very thoroughly at that. It's a drama that kept me hooked from episode to episode. One of my best historical drama of the year.

In conclusion, I loved this drama not just because it's a political drama, it's more because of the main couple. I love their chemistry. "The Princess Royal" was worth my time. I didn't regret watching this, except for the wait when it's airing that is. If you're interested in watching it, I'll advice you to ignore the negative comments and watch with an open mind.