The Book of the City of Ladies is just about as straightforward as titles get. On the surface, it is indeed a book about a woman who builds a city that's just for ladies. It's important to note here that Christine de Pizan uses an old French world that translates as "ladies" instead of "women."
She does this because she thinks that only proper "ladies" should be allowed in her city, and not just any old women. The word "lady" means that a person has good breeding and good manners, and Christine de Pizan has little interest in inviting bad women into her city. Make no mistake; the city of ladies isn't for everyone.
This may seem pretty messed in today's world, but cut de Pizan some slack. She was a feminist way before her time—it would be almost too much for her to be class-conscious way before her time as well.