Twilight -- maybe when Celestia told you that there was a book with the answers hidden in the secret library, she was talking about one of the hundreds of books that *wasn't* her personal diary. Just saying.
Rarity -- castles can't talk. They also don't understand when you talk to them.
Applejack and Rainbow Dash -- the Bee Stare would be a lot more daring if you weren't in freaking hazmat suits. x.x But why was the mailman moonlighting as a beekeeper?
Whoever the writer was -- the Pinkie Pie gag would have been a lot funnier if you'd let the audience think that she'd been assigned to 'ring the school bell all week' and not had her add 'nonstop'. Then it would have been funny when she found out she was only allowed to ring it continuously for five minutes.
Anyway, I knew the phantom was Pinkie Pie as soon as it gave Angel a bowl of carrots. Although from the teaser at the end (or is it?!) sometimes the phantom was actually Zecora. So, really, everypony was in the damn castle.
I liked the Rarity and Fluttershy parts more than the Applejack and Rainbow Dash parts, which were really just the two of them being complete scaredy-cats. Or the Twilight parts which pushed 'yay books!' waaaay too far -- to CMC cutie mark levels. Pinkie Pie was barely in it, so doesn't get to win by default this time.
tl;dr: It's another episode like the season premiere. I think it was a little better than the first two (since nothing in particular made me want to facehoof despite a lack of hooves), but still solidly in the '2' range (out of 3).