I believe, the strongest point of this drama is that it's not draggy. There's always something happening on the screen because the story has so many things to tell you.
It features court intrigues, the work of government bureaus, rising to high positions from the lows, the life of impoverished aristocrats and some commoners, small bandits gangs and organised crime organisations hiding in the dark. And, of course, as every self-respecting kdrama, it has a love story, too! ;^)
Naturally, if you're interested only in one of the aspects of the story (for example, romance), you will be bored watching other points of the show.
Supporting roles aren't outshined by the main characters. All the characters are very diverse (government officials, bandits, gisaengs...) and all have different personalities and looks.
The balance between serious and light-hearted moments is nice, too. Not being gloomy and having funny moments helps the drama to hide some illogical moments of the plot. (For example, the FL sometimes is TOO STRONG running with all those heavy jars full of liquor). But small mistakes didn't ruin the whole story for me. It's not that this drama is aiming to be epic. Instead, it's very self-aware: it often tricks you showing a cliché drama moment but then turns it to a joke.
Music and visuals are also on point. The scenery is just beautiful. The costumes, the landscapes, the colours - everything is very appealing. I was blown away each time I saw some of the fabrics or interior.
However, the pacing of the storytelling is not balanced. Close to the end everything becomes very intense in contrast to the lazy beginning. Idk, maybe that's how it was intended. It's nor bad nor good for me.
I guess, the most charming trait of the show for me was its embrace of Korean traditions. As this drama's main focus is not about royal kings and queens they give you a peek at the life of common people. There were things I've never seen before in a historical kdrama.
Previously, there were some period dramas that the audience found upsetting ('Mr. Queen' and Joseon Exorcist'). So I loved the producers’ attention to small details.
Overall, there was a lot of effort put in this show. I recommend it to anyone who is interested in history or Korean culture in general.
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Now a little bit about my personal experience with this drama.
As I understood from reading the comments, many people were disappointed with the main couple. As I'm nor Yoo Seung Ho's fan, nor Lee Hye Ri's fan, the main leads weren't the reason I picked up this show. They were okay for me.
To be completely frank, that's the first Yoo Seung Ho's role I liked. It's not that I've seen him a lot before, but still. His character and the SFL are my favourite here! Every week I was craving to see the 2nd couple interaction. But I was played :((((
Anyway, I will definitely watch this drama again!!