A tumbling kaleideoscope of smooth transformations, resolving as plot lines, involving true love, and with a girl's age a significant parameter. Yes, there's the moving castle, one of several portals to a subjective world, all of which intersect near a flame (voice by Billy Crystal in the USA version) that contains a loved one's heart.
Overlay that with an ongoing, violent, and stupid war (which all truely powerful wizards are trying to end), and you've got the makings of a fantastically convoluted plot. Some critics have disliked the film for precisely this reason (plot complexity), but I don't regard animation as any less a medium for plot density than say books, with their Tolstoy, i.e. "cartoons for children" remain a sophisticated cultural conveyance for messages of arbitrary depth. Disney was good at it too. Fairy tales have always been deep.