I read this when I was young (9-10) and mostly remembered scenes from the 2nd book, but I also remember it having an unexpected and much darker ending that differed from the typical YA fantasy that I was used to. The feeling, though not the exact story stayed with me, and 30+ years later I searched for and reread it. The ending of the series was still quite unexpected and moving.
As a fantasy adventure it's a bit simplistic, and the 7 citadels are somewhat repetitive. But it's the interludes where the characters evolve, build their relationships, and slowly reveal the mythology of the world that seems to be the real focus of the author. She has something to tell us about the human experience, human worth, resilience, and personal growth.
As fantasy adventure novels I would not rank the 1st and 3rd books highly , but I did enjoy book 2 and book 4 is quite good.