I stayed up entirely too late reading this book.
The minions of the title castle is eager to greet the new Master. After all, the Board has already given a pile of extensions and has indicated there will be no more, but the ravens have said the new master is finally coming.
Turns out she's rather small. And young. But Molly can make herself invisible and is ready to tackle the tasks to become the Master. And has the invitation. (Which, we, but the minions, soon learn, she took.) She meets the minions. Besides the guardian, there is Edward, the animated suit of armor; Cook and her son Angus, both Minotaurs; Pins, an animated stuffed figure; the goldfish; and Serenissima, the steam offspring of a djinn and a woman with just a drop of mermaid blood. And it's a small rather rundown castle with, fortunately, a library where she can hunt for spellbooks to pull off her stunt.
It involves altering a spell from targeting a cow to a donkey, weeding a garden, talking to bats and moles, someone being caught in a block of ice, a tower no longer belonging to the castle, putting a sea serpent in the moat, worms, and much more. It manages to be very funny -- at one point, someone is vengefully baking scones -- and very dramatic.