Queen Seon Duk (2009)

=> Both are dramas set in Three Kingdoms of Korea
=> Both main characters eventually became rulers
=> Both main characters were born with a bad omen, thus they had to live away from the palace in order to be not killed
=> Both main characters returned to claim their title as prince/princess when they got older.
=> Both main characters have an older sibling that loves and protects them

Both involve cross dressing women who are good fighters. They both involve a strong political story with cunning women. They both involve women who are secretly from very powerful backgrounds with the potential to change their countries drastically. They both have love triangles, though Queen Seon Deok focuses less on it.

Set in a similar time period. Starts with parental story then moves to children after a few eps. Involves sisters and royalty. Complicated romances mixed with family secrets and political intrigue. Length.

Though both sageuks have different target audiences in mind (they have differences as if you want some smart political talk you should watch Queen Seon Deok rather than Moon Lovers) they have sone similarities:
> lots of male fanservice (hwarangs in Queen Seon Deok, princes in Moon Lovers)
> the female leads love one man first then she turns to love a man with a darker past (plus both men wear black most of the time, I'm talking about Bidam and Wang So, Kim Nam Gil and Lee Joon Gi Yeah!)
>both the second men the female leads love have mothers that are obsesses with power plus those mothers abandoned those sons too
> both evil mothers have other sons they prefer over the son (the men loved by the female leads) they abandoned without realizing that their abandones sons are the smartest and their closest ticket to power