This manga is weird. The synopsis should have given that away, but the weirdness goes beyond merely the plot. What I find most peculiar about Ryuuguuden is its tendency to switch genres and moods every volume. The first volume is a whimsical supernatural fantasy, then the second turns into a sort of post-apocalyptic science-fiction, and by the third (final) volume, it has become a time-traveling super-powered action story, complete with light sabers. Don't get too attached to anything (and I do mean anything) in the story, because it will change.
The artwork is sparse. Screentones aren't used at all, and shading is done very minimally. Backgrounds are bare or excluded entirely. This is disappointing, because so many interesting directions could have been taken with the manga's visuals. There are a lot of ways to make an underwater fantasy castle look amazing, yet Ryuuguuden took advantage of none of them. The character designs, however, were clearly given a lot of attention. They're unusual, interesting, full of detail, and loaded with character.
Ryuuguuden is solid, and ideal for readers who like their manga quirky and dynamic.