So, what have I just watched?
In a nutshell, a saeguk for kids. Take a Disney film, split it into several episodes, dress the characters in hanboks and you have this drama. It mimics The Moon embracing the Sun - without ever nearing its intriguing plot or romance - in almost every aspect except for the detail of a girl dressed as a man despite the fact that she could never in a trillion years be mistaken for a man. To keep the parallel, we could say this is a fluffy rom-com dressed up as a Saeguk.
The romance is so cheesy I welcomed the politics with a sigh of relief. Everything happens too soon and... too much, depriving my otherwise romantic heart of the needed suspense and, yes, what I consider a must for romance: trepidation. I can't say I am an expert in historical dramas, but the little I know is that almost everything the two main characters do and say here is totally implausible: stroll hand in hand in the palace courts without anyone seeing them? Right. This is just one example out of dozens unlikely - no, impossible - situations. Unlike other reviewers here, I believe the second half of this drama to be a lot better than the first, with at least some plot developments, albeit rushed, at times.
The real saving grace of the show is Park Bo Goum. He clearly is talented and a pleasure to watch. His character is too good to be true, but well played out and multifaceted. On the other hand, I was a little disappointed in Kim Yoo Jung's performance. I know she's very young and has a lot of potential, but she didn't manage to make me feel a thing. By some camera angles, it was clear she wasn't looking at her partner when supposed to do so, and the result was kind of ridiculous. Her character is neither particularly brave, nor very strong or intelligent, so what's left in the end is a super nice crown prince who falls madly in love with a sweet pretty thing because she's a sweet pretty thing. I even came to prefer the appointed crown princess, she was refreshingly spunky.
The music is a collection of pop songs fit for everything and nothing. See above for the age target.
In conclusion, this drama had some real potential, even when the plot twists are predictable, but the final package is fluffy at best. Can be marathoned through for an overdose of lovey-dovey chirping.