@Aliyyah Rusdinar
I want to add somethings from what Hessa said. Taejo (the king) is the first king of Goryeo. He built the nation by unified three powerful kingdom in Korea that time (Silla, Baekjae, Goguryeo). As a newborn nation, Goryeo was very unstable, thus, Taejo use the royal marriage as a way to gather powerful families and stabilize the nation. History record Taejo has 29 wives and 34 childs (25 of them were sons). In Goryeo period, unlike Joseon, there isn't one queen consort. All of the queen married to the king is queen. Therefore, Taejo has 29 Queen. But only 2 got portrayed in this drama. Fascinating right? All of the queen has the same status and rank. Thus, every eldest son of the queens could become the crown prince. The only thing matter in this period is the family background. That's why the 3rd look rather superior than the 8th. That was because the 3rd's mother's family is more powerful than the 8th.
Actually, all of this was informed in the special episode of moon lovers (episode 0). But as far as I know, no one subbed this yet.