Caught up with it in a couple of hours and so far it feels like a worse Gate in pretty much every way.
The biggest criticism I think I can throw at it is that there are no characters. I honestly can't remember the names of any of the characters, it's just "the japanese diplomat", "the good king", "the soldier", "the evil king". They all feel like just entities that inhabit the setting rather than the characters who shape it.
And I believe the main reason for this is the pacing, it's way too fast, it feels like it just wants to get to the Modern vs Fantasy action as fast as possible and as a result there's no POV characters on either side to develop the nuance of the conflict. Evil nation does bad thing, JSDF stomps, none of the fantasy people believe it, rinse and repeat. There hasn't been any questioning of how and why Japan got teleported and we haven't seen a single Japanese citizen.
The art is ok, lots of weird expressions and angles and a lot of stiff looking characters but vehicles and scenery are mostly well drawn and the 80s manga aesthetic gives it a certian charm, tho it looks so old I wouldn't be surprised if this was an old forgotten manga being republished.
That's not to say it has no potential as it has hinted that japan might not be the biggest fish in the sea but it's overall poorly executed in my opinion.
So in sum, interesting concept and setting but rushed pacing and lack of any significant character make it all seem like a shallow modern vs fantasy arena. If you have a thing for this type of realistic modern/fantasy dynamic you might enjoy it as a form of junk food and I'll be reading it for a little longer to see how it goes, as I enjoy the concept myself, but as of chapter 21 it's a 4/10, maybe closer to a 3.