This novel has a lot of reviews already, but I like it a lot so I will give one myself despite everything.
Writing Quality: So far I've seen no mistakes, and if there are then they're too few to notice. Great usage of synonymous and English well written.
Stability of Updates: Amazing, this novel has almost 400, 399 by the time I make this review. I caught up when it was at about 60ish and in a few months it reached up to 400, so yes it has a great schedule and the author is serious about it as well. Definitely a novel that is going to finish and not be dropped halfway.
Story Development: You can divide this between the actual story and the harem's development since it is a harem, and a quite big one at that. I don't know what to say here. The actual story is amazing, it is what keeps hooked up, although the harem part is not bad (it leaves a lot to be wished especially since I'm more of the vanilla type, but that's my opinion), I still prefer the actual plot side of the novel. More than the Kingdom building, I would call it Kingdom toppling. Despite the fact that he's indeed building a Kingdom within the already existing one, what's fun as hell is the procedure and the way he's destroying the royal family as well as the aristocracy within the Empire, that's what got me hooked up...
There are social and political games but they're not too deep for an uncultured person to understand and the fact MC plays the game when he could just end it with his almost unlimited power (literality, MC is OP but like way too strong, he has rivals though and the fact that power doesn't play a major part in their rivalry but actual scheming is what makes the immortal part work for me since they do act like people that has lived for thousands of years) is also a plus.
Character Design: This is another plus for this novel. Every character here is original and stuck to itself to the core. You won't find people betraying their characters for passions or lust or whatever BS emotion here. MC is an immortal Sage that simply decided to live his last life as an immortal in debauchery before ceasing to exist, and he sticks to that to the very end. Every single woman in the harem has her own personality, emotions, some have relationships etc... And they play the part really well, the enemies or antagonists is one of the best parts of this novel. None of them is an idiot, they are scheming, crafty, some are very powerful, some are haughty yet cautious, you won't find the typical stupid young master and if they're here then they will be drowned in the middle of the scheming of the higher characters rather quickly.
World Background: Nothing to say here, 4 stars since the world isn't really that big compared to others I've read but it is explained in a simple way and it is good enough to serve as a proper description to imagine the scenario but still, it could've been better in my opinion.
Now my personal opinion and it has light spoilers so you're free to keep reading, though I think what I wrote above should be good enough for you that is reading this review.
I like single pairings but I also like harems, everything as long as there are honest upfront and no cheating or lying (basically, I don't like harems where the woman doesn't agree to be in it, unless... She's a villain and is getting what she deserves) I rather like vanilla too but I don't reject some spiciness either. This harem's every girl is unique, the way they're conquered (this word can only apply for some of them since there are some who he forced himself into) is unique as well, I've read so much already that normal things don't catch my attention and this gem is one of such.
I don't like MCs that are OP and just go around destroying everything left and right, this one if perfect: What's better than an OP MC? and OP antagonist that can fight him to a standstill while they keep rivalling each other, like L and Light, everything went to the rubbish bin when L died, in this case, it is similar. I believe it will lose a lot of charm once the Empire is conquered but still, that's within the author's plans as much as I can tell.
Let's resume this: If you don't like women's opinion being disregarded in the relationship then this is not for you (it doesn't happen with all of them, but it happens with most) MC will just force his way or conquer them whether they want or not, one way or another. The good point is that he moves his cards well in order for the grieved women to not stay like that forever.
If you don't like harem this is not for you.
If you don't like MCs that are OP but don't destroy everything and take everything they see by force but rather use their mind, then this is not for you. (MC is not only OP in strength but also in intelligence and the later is the resource he uses the most be it to conquer women or to destroy his enemies, strength is only used when extremely necessary)
if you like political games, this is for you.
If you like Netori, this is for you.
If you're R18, obviously... This is for you.
And sadly, if you're into rape, this is for you. No matter how subtle compared to others, there is no denying that this novel has rape. Forcing a woman to have sex against her will regardless of if she's good or evil or if she ended up liking it or not... Is rape. I do the author a favour by putting it here last since I don't consider this being a point bad enough to deter people from reading this.
Peace.