I think Twilight's Kingdom is massively overrated. It remains my least favorite two-parter and among my least favorite episodes in the show.
I've seen it probably about 2 or 3 times, and I can't say that I'll ever care to watch it again.
One of the things I hate about it is how purely incompetent Celestia is. McCarthy has not treated Celestia well since Lesson Zero. Nearly every single additional time she's written Celestia, it's pretty much just for the express purpose of dragging her into the ground while simultaneously making Twilight look better. Where once Celestia was believed to be the equivalent god among the fandom, that perception has been utterly shattered, and McCarthy is entirely to blame.
In Twilight's Kingdom, McCarthy took things to a whole new level. She painted Celestia as easily the most incompetent ruler I have ever seen, bar none. Celestia's decision-making in this episode is flat-out ridiculous and certainly does not give indication that this character has been ruling a nation for more than a milleniuum.
For starters, Celestia's first major blunder is sending out a recently-reformed villain to stop a new villain, without any backup plan whatsoever. Without any good reason whatsoever, she put complete and unwavering faith in Discord to stop Tirek. Because how could that possibly go wrong? It's certainly not as though Discord hasn't proven himself to be entirely trustworthy, you know, by the way he deliberately misled the mane six in Princess Twilight Sparkle and how he deliberately pitted Twilight and Cadence against a giant worm that could have killed them. How could Celestia have ever expected that Discord would betray her and instead help Tirek gain power, effectively rendering her at fault for the fact that Tirek was able to drain magic from just about every single pony in Equestria? I mean, come on, you'd have to be a strategic genius to have seen that one coming!
And of course, Twilight ends up looking all the better after this massive blunder of Celestia's, because she had offered to stop Tirek herself before Celestia chose Discord, and Lord knows that if Celestia had actually allowed Twilight to do so, Tirek never would have been able to gain power and would have been quickly been sent back to Tartarus. So yes, score one for Twilight, and minus about 10,000 for Celestia.
But unfortunately, Celestia's poor decision-making doesn't stop there. I truly, truly wish it did, but unfortunately, McCarthy was not at all finished running her into the ground. Because once Celestia realizes "I dun goofed!", her very next step is to deliberately render the three ponies who stand the greatest chance against Tirek utterly powerless, and transfer all of their power to Twilight to keep it safe from him.
Why....just....why?
This one just makes no sense. At all. I mean, thinking logically, if you're trying to protect alicorn magic from Tirek, wouldn't transferring it all to a single pony just make it easier for him to get to it, because he would only need to draw from ONE source instead of four? Is there any good reason why Celestia decides to do this instead of taking a stand against Tirek alongside the other three alicorns, which is quite clearly the best chance they had to stop Tirek at that point? And another thing, Celestia makes this decision under the belief that Tirek will not know of the existence of a fourth alicorn, and thus the magic will be safe. But this makes no sense, because Celestia has no reason whatsoever to believe that Discord would not have already told Tirek all about a fourth alicorn, which, of course, he does. And lastly, once their magic is transferred to Twilight, what exactly is she supposed to do then? Celestia doesn't even give Twilight any further measures for keeping it safe--she doesn't advise her to go into hiding or anything. So what the hell makes her think that Tirek isn't going to find her? I'm sorry, but under no logical circumstances does this plan make any sense whatsoever.
But unfortunately, that's still not the end of Celestia's incompetence. Because she then advises that Twilight not tell her friends about this magic transfer under the impression that doing so will simply get them captured by Tirek and pressed for information about Twilight's whereabouts. And Twilight actually follows this advice, but guess what ends up happening? THE REMANE FIVE GET CAPTURED ANYWAY.
There just is not so much as A SINGLE THING that Celestia does in this two-parter that makes even a SHRED of sense. She is portrayed as more incompetent than she's ever been, and it's all for the purpose of giving Twilight enough power to face off against Tirek later on.
As for the fight scene itself, it's certainly epic. And it's certainly darker than anything I would have expected to see in the show. But it's completely and utterly worthless to the plot. Aside from getting Twilight's library home blown up, it really didn't contribute anything to the plot. It was just there to fill some time until Tirek presented Twilight's friends to her and offered them up in exchange for her magic.
And Twilight was definitely stupid to give her magic up, I might add. Because, just like Celestia, she had no backup plan, and it was clear that the best chance Equestria had to not end up under Tirek's control was for her to simply keep on fighting until she killed him. And once Twilight decided to give it up, Tirek had no reason whatsoever not to kill her on the spot, and it still makes absolutely no sense to me as to why he did not do that, other than being too dark for a kids' show, of course. Still, though, the pony who had just dished out a serious beating on him had just been rendered completely powerless, and Tirek had sent the other three princesses to Tartarus for pretty much nothing. It makes no sense for him to suddenly drop his vindictiveness and let Twilight and her friends go instead of just kill them right then and there, or at the very least, sending them to Tartarus as well so that they no longer posed any threat to him whatsoever.
Overall, the writing in this two-parter is just awful. It's terrible. It's the worst I've seen in any two-parter. I just hate this episode.