Let me begin, this storyline does not match truly the real one written in history, yes on dal was a fool and a bumpkin, his mother was blind, he didn't have a general father, when he met the princess his life got turned up down and became a general who fought for Goguryeo and was known as his strong will and power. On the other hand, the princess was not a soldier nor an assassin so why to damage her reputation. Yes she has knowns martial arts, she loved on dal to get off from marrying a noble man while she didn't want him when she was young. So afterwards their marriage was established out of love and care. On dal after becoming a general of Goguryeo crossed into many battles to save his country and was a hero then he died while defending the borders against Shilla army by an arrow on 590.
Returning to this drama, The four main leads, go Geon, on dal, Pyeongyang and mo young portrayed respectively by Lee ji hoon, Na in Woo, Kim so hyun and choi yoo hwa did their role with utmost sincerity and braveness but the script and the flow of the drama didn't fit them, I liked the chemistry between The FML and FFL and the SML and the SFL, they were awesome together but the outcome was a bit strange so go Geon that he was the princess trainer from her childhood love her for years but she didn't blink an eye and he didn't have a chance to act because of her love for on dal and her grudge against his father go won Pyo. Nonetheless,, Mo young who was a silla spy, became attracted to master go who was twisted and obessed by the princess, she had nothing to lose and she keeps confronting him while he was in pain and a dire situation. Go Geon could not forget his love for the princess even if he was grateful to mo young care and help aid so what a man. While On dal only cared about the princess and died protecting her which was heartbreaking and same as go Geon death.
So this drama is far to be believed and one thing that phenomena of masquing the spirit and reviving it again to make FML alive is unreasonable and they should not have include it in the script. On dal death should have been kept as it was and replacing it with something else would be more ominous.