Two of them in the same time period and political situation.
My Dearest have a love story. And we can see also small fun scenes. But Fotress more about king and him ministers with sad and cold atmosphere.

Mythical-mystic things
Evil creatures
ML's brother wants to kill FL
Love story with reincarnation
Both of dramas have similar atmospheres

ML looks like a excelente husband. But he hide some things that will destroy their marriage.
FL have some relationships/friends but she didn't know they have connection with her husband secretly.
Both of them feels same in term of cinematography, music and feel 'secret'.