I am so madly in love with the this Drama  - The Flower in Prison - plus i am into with the actors and actress - Go Soo my one of those fave actors he is so capable of doing action drama and more on Historical one - he is likely same and can be a good successor of Jumong and King Gwanggaeto :) These 2 kings I love the most... , Seo ha-jeon - played an important role as King...  Choi Tae-joon - Though little spotlight in the drama but he is a good actor that can compete if given a chance.
The most pretty lady in the Korean Historical Drama - none other than - Jin Se-yeon - she is very good and excellent in acting, I even watched her dramas - Historical - Grand Prince, Queen: Love and War and the likes.. she is stunning in her own way... 
I know it is late for me to watch this drama aired in 2016 - it is because I checked this out late too.. :) When I saw her in the earlier drama's i mentioned, i became addicted to her...so without hesitation i watched right away the The Flower in Prison considering Jin Se-yeon is the lead role actress - I know it would be a great drama.
It was a little bit awkward when the King felt strange feelings for her and found out she was her sister... :) you know what I mean...
Go Soo as Yoon Tae Won - is my love :) I like him - he can be Jumong and King Gwanggaeto roles if given a chance...  :) because of his manly looks and beauty :) ... i just came to know him in this drama, that is after watching it - i went through all his drama and movies where he was the lead role. 
Overall - the drama is EXCELLENT!