So all of the Mane 6 get trapped in the castle, and weird things happen. There's a pseudo-mystery about who's causing these weird things, though the answer is pretty obvious. Hilarity ensues. No, really, it's hilarious. Just looking at the jokes, it's already one of the show's funniest episodes. There's a new writer on board here, and he's really good at comedic writing.
In addition, he's incredibly good at writing the characters. They're put in groups of two, and they have strong chemistry with each other. In fact, the character writing here is arguably better than anything in the past season. They're all at their most entertaining, perfectly playing off of each other and the environment to comedic effect.
And of course it's well-animated, has good background music, etc. But, with that being a constant in this series, it's to be expected.
Overall, it's a hilarious episode. Not much more to be said there. The characters are great, the setting is great, and the writing is great. Just an all-around great episode.