Backtracking in a non-Metroidvania game risks killing it completely in most cases for one reason or another. A lot of times, it’s just padding, and KH2 gives the impression that’s what the game’s second half will be like.
Mulan-land wasn’t very fun to begin with, and going back there doesn’t change much, so when you first get to Beast’s Castle again, there isn’t a lot to hope for. Sure, it was a challenge the first time around, and the story necessitates it, but it still leaves you feeling a bit bored. Until the end. This is where Sora first fights Organization XIII (outside Castle Oblivion, that is), and it’s a fight to remember.
In fact, it’s one of the game’s most noticeable difficulty spikes, and depending on your playstyle, the potential need for grinding beforehand is enough to make it memorable. Xaldin is a whole new kind of boss, fast, deadly, and resilient. The encounter makes you think on your feet, throwing in some Mega Man-style pattern recognition and avoidance as well.
And it sets the tone for the game’s second half — darker, harder, and more demanding — while kicking off some major lore dumps as well, where you finally get a slight understanding of what’s really going on.