There are two things that came to mind when I read this book, which by the way is quite moving and beautiful. First, I thought of a castle as the classic little girl's dream. The fact that it is glass makes it more beautiful, but also more fragile. Second, a glass castle cannot be real. It's a lie that people share in order to manage under difficult situations.
The glass castle is an idealistic, dreamy place of her father's fantasy and that perfectly fits what we come to see as his life and how he chooses to live it. At the start of the memoir the unconventional life he creates for his family "works" on the basic level, but as both he and her mother become more unstable, the glass castle of the families lifestyle shatters and the family is living in the opposite circumstances of a dreamy, fantasical place -- it is a living hell on earth that no child should have to endure.