Though one review of this series says it blends science fiction with ancient Korean history, the time frame of this drama is roughly contemporary to the publication of the King James Bible in England (1611). That is why, though the costumes and customs might look medieval to us, the viewer nevertheless will see guns on this show occassionally. (Including a sequence in which the hapless Magistrate Jang Man tries to master this strange new device only to shoot a hole through his own hat.)
The series does refer back to earlier Korean history; some lore having to do with haunted places and UFO sightings goes back to previous dynasties that have there origins if not their end dates in ancient times.
The language of the show is translated in English subtitles to favor Elizabethan (Shakespearean) uses of terms like "milady', 'meseems', and so on although these words might have been on the way out even when Shakespeare used them.