My historical knowledge is not the greatest but I’ll do my best. Keep in mind the historical setting is familiar to those who’ve grown up having to study it (and Park Hyung Sik’s character will eventually become a very famous and influential king).
The queen is still regent because the king was too young when he ascended the throne, and he’s in hiding because, originally, she was worried he would be assassinated since there’s a strong contingent that are against this royal lineage and want to put someone else on the throne. They’re opposed to Hwarang because it’s a foundation for trying to get the various factions to work together in unity and support the throne (which may not be exactly what you want if you’re against the person currently sitting on the throne right now).
Hwarang is based on actual historical figures – essentially elite “knights” who studied art, culture, religion (helped to establish Buddhism as the main religion), martial arts, and sports. They’re also historically considered to be the “beautiful elite” so you can see why someone was like “hey let’s make a drama that’s all about ridiculously pretty guys.” Even though the show definitely takes license, honestly it’s not too far off with the idea that these pretty aristocrats were schooled together (and probably got up to some shenanigans).
Also of note is the “bone rank system” which explains why Park Seo Joon’s character gets bullied so much (since he – pretending to be Lee Kwang Soo’s character – is thought to be half “true bone” and half commoner, and not worthy enough in his lineage to be a true part of this elite aristocratic group). You’ll see the bone ranking brought up quite frequently, so it’s a good thing to understand.
The Obsessive Dilettante 