Rich in plot and story, but the drama should've just gone with sci-fi and fantasy instead of trying to include romance as well. The female lead's character is poorly developed though Park Shin Hye does her best in trying to bring life to the character, and the romance itself has very little basis and feels a bit forced. There are also very few important female characters, with the drama barely passing the Bechdel-Wallace test and the Sexy Lamp test, but failing the Mako Mori test for the majority of the drama.
Other than that, the drama is lit, and definitely deserves to be watched.