I read this when I was pretty young and it stuck in my mind for years: it's a fun world, a kind of Medieval setting where religion is a choice between Order and Chaos, in landscapes governed over by terrifying storms and strange elemental creatures. The main character can talk to cats and she really wants to go to the school for magic teenagers her parents met at. It's a huge menacing castle over an abyss: this isn't a kid who's scared of the unknown.
In retrospect, the world was cool but what I'd really been attached to was how the author writes teenagers. Shar is a special kid, born at a deeply significant point in time, and the novel is about her figuring this out and learning what the implications are while various parental forces try and shelter her from her own significance. They've all got different motives, some good and some bad, and a lot of the fuel behind the plot is the novel's trio of totally plausibly written teenagers determining which adults do and do not have their best interests at heart. If you picture a less classist Enid Blyton writing for teenage Wiccans, you've probably got a pretty decent idea of what this story's like.