Kingdom is a series that people like to hype up these days and indeed has a great premise of ancient Chinese warfare that harkens back to wide spanning stories such as Romance of the Three Kingdoms. However, its poor presentation and repetitive nature causes it to be about as impressive as a second-rate Shonen.

Now, it certainly isn't an absolutely terrible series (though it being rated a 5 and not a 1 should make that obvious). It seems to have some semblance of war strategies and some of the supporting characters do develop as the story goes on to some extent.

The main complaint of the series' execution lies intertwined with both its plot and characters, or more specifically, its main character. Xin is a terrible protagonist as not only is his personality and motivation a typical hotheaded youth wanting to be the best in the likes of Naruto, but the plot armour provided to him is insane for a series that attempts to pass itself off as a historical piece. Not only does Xin outperform and defeat skilled assassins without much real experience, but doesn't even do it through actual tactics and instead just by shouting and overcoming his opponent through willpower as in any run of the mill Shonen. This practically betrays any engaging premise the series could hold as the attempts at strategies and the support characters' roles are made meaningless when the climax of several major battles focus on a kid overcoming challenges by himself without any thought put into it.
It also doesn't help that many of the support characters seem overly interested in Xin's actions or potential without much reason to do so, such as Wang Yi practically entrusting his legacy to him for typical passing the torch reasons within coming of age stories or the young girl having romantic feelings for someone so dense it took them over a hundred chapters to realise they were female.
Now, I can't fully say if the quality has improved since I dropped this work almost 300 chapters in, but if a series takes that long to become more than a typical Shonen in more than explicit content, there's no real reason to view it in any worthwhile light. Yes, some series take a while to get good, but over 300 chapters in is more than pushing the limit.

To get to the point, Kingdom gives too much of a focus to its protagonist and his Naruto-esque lack of logic to the point it heavily damages the series' overall quality. Overall, this series that is supposedly praised for its genius is one with such a lack of logic revolving around the centrepiece that it could be insulting to my intelligence.
If wanting a good manga about Ancient Chinese warfare with genius strategies and very little plot armour, read Sangokushi or most Romance of the Three Kingdoms adaptions in general (particularly the 2010 live action series Three Kingdoms) or if wanting a period drama with memorable and well written characters, watch Rose of Versailles.

Overall ranking - 5/10 mediocrity