Hi, this is my first review ever!
I thought I had to do this for some JoJo's bizarre adventure installment (it's my favourite anime ever) but when I ended this fantastic series I felt that I had to say something on it, I had to tell everyone why I loved this that much.

Story: 9/10
I usually tend to prefer stories with a serial storytelling, but if all the episodes in an episodic series are amazing I'm gonna love it anyways.
All of Kekkai Sensen's episodes are brilliant and enjoyable; it has some horror, some cute romance, a lot of funny moments, a bit of thriller, a bit of science fiction, a great [SPOILER REDACTED] and even a gambling and a fighting episode!
All of the adventures and mishaps that our main character, Leonardo Watch, and his comrades have to face are set in a colorful and mad world to which everyone would give a peek.

Art: 10/10
Kekkai Sensen's artstyle is something fantastic yet unique: the character design (made by Yasuhiro Nightow, the same author of Trigun) is brilliant and describes perfectly all the characters (e.g. the comic characters are drawn in a ridiculous way), the world in which the story is set is somehow similar to ours but where the outer world merges it blends with the first in a way that feels pretty natural.
The animations are always fluid and pretty flashy during the fights and that bit of CGI used is pretty good to see (I'm thinking about the last episode).

Sound: 10/10
The original dubbing is perfect as with every anime produced by Bones (I'm thinking about BSD and Noragami in particular), but the best part of this anime, imo, is the soundtrack.
The original tracks are all brilliantly written by Taku Iwasaki (the same author of Soul Eater, Bungou Stray Dogs, Tengen Toppa Gurren Lagann and many other anime's OSTs), the OP is beautiful, the ED is imo the best ED ever made and the not original soundtracks... are perfect.
Mozart's The Magic Flute, Borodin's Polovesian Dances, Satie's gymnopedies: all of these classical music pieces and many others are perfectly used in this amazing anime and sent chills down my spine.

Character: 9/10
All the characters are perfectly typified for a 12 episodes series (surely they would have been developed even better if the series had been a bit longer) and you can vibe with each and every one of them: Leonardo is amiable and brave, Mary is mysterious yet beautiful and cute, Zapp is a super funny hot headed punk, Klaus is a stoic and brave father figure, Chain is a mysterious girl who perfectly balances Zapp's agitation with her phlegm and Steven is a perfectly written sidekick for Klaus.
All the secondary characters are also nicely written from the fun and mad Aligura to the lucidly insane Femt, from the comical Luozontam Ouv Lee Nej
Amagranoff to the mysterious and clever Don Arlelelle Eruca Fulgrouche and the friendly yet serious Zed O'Brien.
It's an amazing cast that vibes with you and makes you want to be one of them.

Enjoyment: 10/10
I really loved watching this anime: I enjoyed watching Leonardo and his comrades face every kind of absurd situation in Hellsalem's Lot, a city as wild as its inhabitants.
I liked a lot every aspect of this series and I'll totally recommend this to anyone will ask me for a great and funny series!

Final thoughts:
Kekkai Sensen is a brilliant anime series inspired by the entertaining homonym manga made by Yasuhiro Nightow, the same author of Trigun.
It made me laugh, cry and get excited a good bunch of times throughout its 12 episodes and Im pretty confident it deserves a full 10 out of 10.
I'd strongly advice you watch it if you haven't yet, as im sure you wont regret it!