'The King: Eternal Monarch' had potential but wasn't properly developed. I' m giving this show a 6 stars rate because overall I enjoyed it, though; 2 episodes a week weren't hard to watch and they were entertaining enough. The cast was really nice and I really like Kim Go Eun's acting, but I have to say this isn't her best. That's probably because her character is so poorly written (just like every other one) it's just impossible to save it. At first I found myself rooting for her, she seemed really badass and able to look after herself. Didn't take long for her to switch into our typical demsel in distress, though. I don't know how many times we see her cry during the whole drama, but I'm sure it's a lot and I just couldn't stand it. Lee Min Ho's character is just the same as in every drama, or at least that's how it seemed to me. Always playing the saviour and being this perfect, humble, flawless king.
Kim Kyung Nam was great in his role, too. I hadn't seen much of him before and it was nice to do it here, but again, his character was just plain.
The only saveable characters are Woo Do Hwan's. Jo Yeong and Eun Seop lightened up this show, the acting was killing and the writers still tried to take that away from us giving him so little screen time. I'd have liked to see his background story alone and not only his story with the king, but obviously, there really isn't any space to delve into the characters and they just rushed everything, again.
The rest of the characters, like the villian, aren't worth mentioning. So poorly written, no background story, just there to make the ML look heroic and to the FL look dumb.

The real mess comes with the plot, tho. We can endure some badly written characters if the plot is good, right? But this wasn't the case. It had so much potential but it was just wasted. While I was finishing the drama I started feeling like the writer just didn't know how to finish this and tried to fool us with a lot of emotions (most of them didn't work). A lot of things stopped making sense, things that were shown to us in previous episodes that were intriguing just weren't brought up anymore in the whole drama and we were left staring at the screen like fools- wondering where they went.
This happens with a lot of things like the doppelgangers (Luna, especially), the travelling consequences and some characters I won't name to avoid spoilers.

The romance on this drama is just superior and not in a good way. Tae-eul seems to hate Lee Gon, but after he proves her his world exists she's deeply in love with him. The same goes for him, wanting to make her his queen the second they meet. Boy, you didn't even know if she actually existed a few minutes ago, calm down. The kisses are good and I somehow managed to see some chemistry between them, but it wasn't enough. It felt really rushed.

In conclusion, I was entertained with this drama because I only had to watch 2 episodes a week and they ended the episodes in a cliffhanger of questions that were never answered, so I feel pretty fooled. Still, I laughed a lot with Eun Seop and even shed some tears in the ending episodes. Didn't really get to feel empathy towards most characters, though, apart from Woo Do Hwan's and maybe a little for Lady Noh, ignoring the leads.

Give it a try, you may like it, but don't get your hopes really high because then you'll end up really disappointed.