This is a decent fantasy anime that both eschews and embodies many of the issues common to fantasy anime.
For one thing, drawing on ancient Persia as a setting is a very cool choice, as it's fairly underutilized and lends the show a fresh and exotic coat of paint. On the other hand it does fall into the typical anime-set-in-the-pre-modern-era trap of grotesque historically inaccuracy when it comes to numbers and scale: expect big battles with classical era armies somehow consisting of hundreds of thousands of men.
But that's not really a strong criticism, given that it is a fantasy anime, and historical inaccuracies like that don't really have much impact on the overall quality, they are more of a personal peeve. What does detract greatly from the show is how horrible these massive battles look. Every battle scene gives you long panning views of big, grotesque CGI armies, of copy pasted spearmen and knights. It looks absolutely terrible, but fortunately these large battles are used sparingly - when the show recedes in scale and settles into a more conventional fantasy story scale of a small party of people, the animation is no longer an issue, and the characterisation and pacing shines.
The characters aren't groundbreaking or deep by any means, but they do fill their archetypes well and act consistently. The cast is a little oversaturated, meaning some accessory characters are starved for screen time and some early prominent ones become basically side characters, but even in their limited exposure they make enough of an impression to still feel like actual characters.
However the pervasive cancer of boring shonen morality mars any thematic depth this series might have. Arslan discusses slavery with all the nuance of dismemberment. There is a flagrant ethical code transposed from contemporary society and applied to this setting, in order to win the viewers sympathy and support for the protagonist.
While this does not ruin the adventure or fun in this show, it does prevent it from feeling like a really special pseudo historical drama. Which is fine, as it never really tries to be outside of phonetic references to real world historical locations and titles.
I would definitely recommend this series to anyone who enjoys historical fantasy with minimal fantastical elements, especially if they have a love of romanticism in their fiction.