Honestly everything in this anime is amazing. Hell it might even be anime of the year for me, but I digress.
The story is quite interesting, it sets on a world where kings are ranked based on many things: population, economy, the king's leadership, etc.
Among the thousands of kings out there in this fantasy world, King Bosse ranks 7. Renowned for his strength, everyone expected his son to be as strong and big as him... though those expectations were not met. Our adorable (and incredibly cute) first prince Bojji is both mute and deaf, but he makes it up with his amazing ability to observe. He meets with Kage, who was once part of an clan of elite assassins, and begins their wonderful friendship with each other.
The world looks like any other fantasy type-beat but it's significantly different than what you would expect. Why was Kage's clan cursed to look like that, why it was eradicated, who created the king's ranking system, and much more. Its very indulgent to slowly watch the answers getting revealed one by one, while also creating even more questions.
Animation and OST
Don't be fooled by the childish style, because it includes everything you could ask for, badass action, unique monsters, and quite a lot of violence. Both the OP and ED are an absolute wonder to hear - King Gnu performing for the OP and yama performing the ED with its fantastic story book animation style.
Tldr: it made me cry in just two episodes, that's how good it is. Watch it.