As I understand the term, “fusion” sageuk is not based on actual history (which was cruel, bloody and terrifying for just about everyone) so the writers can incorporate more humor and more true romance (as opposed to arranged marriages). It’s my 1st time watching any sageuk beyond the first 2 episodes for the following reasons [spoilers!]
1. The main reason I continued to watch beyond Ep. 3 was the Character of Shi-yeol. I have never seen the actor Kang Hoon before, but I am now watching many of his prior dramas: Little Women, etc. He carried the show forward.
2. For the first 10 episodes, there was a continuing mystery as to which of the scholars was the real lost prince, which one was the “Watchman” and if there were any other assorted “royals” who might have a claim to the throne. The Watchman “reveal” was one of the best written, directed and choreographed scenes I have ever watched – and re-watched. Truly Epic (and bloody).
3. Much of the drama was full of many side characters with conspiracies, betrayals, spies/counter-spies but at about the 12 episode mark, the puzzle pieces began to align and the viewer understood who was friend or foe and (except for one minor character in Ep 18 – a truly wasted 4 minutes) there were no major surprises. In other words, the writers carefully wove the various storylines together and there were no “WTF” moments where a character totally forgot about a prior conversation or new facts were suddenly inserted to move the story forward.
4. Happy Ending for the entire Ilwahon family: Dan-oh, Kang San, Yu-ha, Shi-yeol, Naju, Yook Ho and Hong-joo
5. I hate star-crossed lover stories usually, but the slow-burn romance between Shi-yeol and Hong-joo was beautifully written and because of their backstory/history, it was easy to see how their connection grew and evolved. In my view, they outshone the supposed Main Couple whose primary claim to romance was a childhood connection. Anyway, very happy ending for both couples, so good job writers.
6. The sword fights (and there were many) were well choreographed – outstanding stunt work. The production team did not spare expense in filming the final outdoor scenes both within and outside the palace.
7. All of the bad guys died 😊

A few negatives:

As w/most k-dramas, the ending (last ½ hour of Ep. 18) was rushed as everyone’s story was wrapped up with a neat bow in a two-year time skip. However, the reconciliation between Shi-yeol and Hong-joo was a series of small scenes – I was frankly surprised at the writers/directors because they shared short “sketch” scenes which may have been in the novel, but as I said – rushed. Not even a kiss or hug when they reunited. ☹☹

Lots of blood spilled but the most graphic scenes involved the bad guys, so easier to handle.