I'm glad to hear you're introducing both Lego and fantasy games into your niece's life.

The only themed Lego I had as a kid, as distinct from generic Lego which was more common back then, was the first castle. I bought it with my own money that I'd earned from walking dozens of dogs!

♥ https://en.wikipedia.org/wiki/File:Yellow-Lego-Castle_375.jpg ♥

I don't have it any more but I still think it's one of the best Lego sets ever.