Upon first seeing this title, I assumed that it would follow your generic fantasy!world anime.
Having rewatched it now for a third time over the years, I have to say its probably the best in the genre, especially in terms of world building and plot development.
The anime adaptation of the original novels focuses on Yoko Nakajima. She starts out as a very freaked out Alice in Wonderland who really just wants to go back to her home and school in Japan. But over the course of the first story arc (episodes 1 to 13) we watch her develop into a strong heroine. It's one of the many things I appreciate about this anime; the changes and growth of characters that takes place over the span of several episodes. I would start out hating certain characters initially (this even happened with Yoko, TBH) but at a certain point I'd realise I couldn't be more pleased with how awesomesauce they'd become
And as with characterization, relationships also change; friends may not always remain on good terms, but the reverse is also true. Ultimately, even while it may take place in a different world, the story the Twelve Kingdoms tells revolves around human nature. Don't get me wrong - with world building on this scale, there are a lot of interesting fantastical elements like Kirin, Hanjyuu, Taika etc. At the end though, the focus is on the people of the Twelve Kingdoms, who in many ways are reflections of ourselves.
My favorite story arc is probably A Thousand Miles of Wind, The Sky at Dawn (episodes 23 to 39) where we see characters introduced over the course of the anime band together and suddenly we can tell, its the beginning of an era that will change the world and generations to come.
The only drawback is that there are just 45 episodes. I'm not kidding - there are 12 episode anime titles that seem to drag out more than this series. With four main story arcs, the anime is actually pretty fast paced and I wish there were more tales about the characters from the novels adapted into another follow up sequel series.
To get a better idea of the series, october_26 and painless_j posted up their experiences watching it for the first time, here and here which are definitely worth reading (especially for new viewers). PJ also posted up fic recs on her lj today for those who've watched it already :D
+ Link to translations and scanlations of novels etc. on LJ \o/
Title: The Twelve Kingdoms // Juuni Kokuki (2002) [ MAL | ANN | IMDB ]
Genres: Adventure / Drama / Fantasy / Action / Historical
About: Nakajima Yoko is your average somewhat timid high school student. One day, a strange man named Keiki appears before her, swearing allegiance. Before she could properly register what was happening, demon-like creatures attack Yoko and her friends, after which they are pulled into a different world. A world unlike what she has ever known.
Separated from Keiki, Yoko and her friends discover that as "Kaikyaku" (people who come from Earth) they are hunted fugitives and must do whatever they can if they wish to survive in this new world and figure out the reason why they were brought there.