A bit random but, I just realized that the overall reception for this manga is much bigger than what I thought.
Everywhere you check the available volumes not only have a high amount of reviews (Especially Amazon, where it rivals and surpasses even popular Shounens) but the overall rating is also really positive. On top of that, the Kindle edition of the recently announced Vol 4 is frigging Top 2 in the
Graphic Novels Bestseller List, only surpassed by the also recently announced Made in Abyss Vol 12. (By the time I'm posting this)
Also, this keeps getting more and more popular on the YoungChampion website, where it has more than 80k likes (It seems the only manga that surpasses it is the ultra popular "The Dangers in My Heart", but I don't exactly use this site)
To be honest this comes as a surprise to me because the reception for the LN isn't even close to this.
It wouldn't surprise me if this (Well, the LN by proxy) gets an anime adaptation in the nearby future.
(Of course, until we have actual sales numbers, it's hard to tell how popular it actually is, but from what we have, this 100% seems to be a hit)
I haven't read the LN or original VN, but if I could give my 2 cents just going off the manga, I think it's a combination of a low-stress, feel-good overarching plot combined with exceptional fight choreography.
Folks here have mentioned the art, which is pretty good, but not on the level of something like Berserk or Vagabond. Rather, where I think the manga truly shines is the thought put into the fight scenes. In most run of the mill isekai or shonen manga, the actual physical details of swordplay are glossed over. There's a flurry of slashes as the combatants move faster than the eye can see, they zip past each other with cuts appearing on their bodies after the fact, and so on. There's a bit of that in this manga, such as with the Knight Commander's battles, but for most of the other fights the artist seems to take care to portray the ways a swordsman would actually move and the various things he'd take into account in terms of the environment, the position of every part of his and the enemy's body, and so on.
These strengths are most evident in the fights Heimbritz has had. In his first training match with Beryl, the viewer is treated to the exquisite detail of how Beryl flipped his wrist to deflect Heimbritz's swing by catching the latter's blade with his own crossguard and then, in the same motion, bring his own blade overhead to bop Heimbritz. And then, in this current battle, we really get a sense of how brutal and no-holds-barred the evil assassin guy is, and just how Heimbritz can match him: Again, the artist depicts in detail how the combatants fight not just with their swords but their entire bodies, grappling with each other on the ground, breaking an arm, then stabbing a leg with a piece of wood. This makes for a much more engaging fight scene than you usually get in manga like this.
All this is combined with a story that itself isn't really as brutal and no-holds barred. Unlike Goblin Slayer, or much less Berserk (I love Berserk, so don't take this as a criticism!), the setting of this manga doesn't seem as utterly malevolent or callous towards its inhabitants. Indeed, Beryl's profound decency shines through constantly--the reader just has to feel a twinge of affection when he covers up the eyes of the girl who tried to betray and steal from him so a "child" like her doesn't have to see a scene of horrid death.
For a lot of people, especially older ones (like me ;_; ), I think that adds greatly to the draw of this manga. When you're old and your body is giving out, it's nice to withdraw even for a little bit into a fantasy where an old guy can still keep up with his younger proteges, and even more than that, where his compassionate and, for lack of a better word, soft nature is praised and tended to rather than coldly snuffed out. Combine that with extremely well planned battle scenes and you have a recipe for something with at least slightly wider appeal than the typical shonen.