An image of a perfect flawless world. In the quote, and in the story generally: there was a similar representation of how Rex is going to upbring his children in a flawless glass castle with his super-genius skills, along with Rose Mary who had the illusion of being a great famous artist whilst she's just a narcissist who has sympathy for rats and canned food more than her own children.

Self-Sufficiency or Self-Harm?  Even during their hardest times, Rex and Rose Mary refused to become charity cases and refused their children's help in their adulthood. That's how this image comes into place, they were independent to desperate measures, to the extent that they didn't even go to hospitals. Rex had checked out Jeannette "Rex Walls Style" from the hospital when she was burnt just because he didn't trust the system, when they fall ill or injure themselves, Rose Mary prefers to treat the wound at home rather than cater to what she considers a false need to visit the hospital. They even passed on to their children that they should not rely on society or doctors or anything else to help them through life. 

Jeannette, since young age, has been persevere to survival and achieving her goal of a coherent life along with her sister Lori. For that reason, Rex calls Jeannette Mountain Goat. The nickname refers to Jeannette's endurance in face of trouble. Like a mountain goat, she is able to climb mountains without losing her footing. The image shows a person who has almost fallen from the edge; it's an epitome of the kids' lives. After all what they had been through in life with their unordinary parents, they managed to pull up and through.

Rex made lots of promises to his children, such as making sure to take good care of them, build them the glass castle and be a better father figure by stopping drinking. Rex seemed to care a lot about his kids, he just didn't understand all of the responsibilities of being a parent. He tried his best, but ends up spending money on alcohol and not food for his kids. He started spending more and more nights drinking once they move to Phoenix and he lost his jobs. He pretended that he is rooting out the mob, but in reality he was getting drunk. To rex, the glass castle represented hope, however, to Jeannette it represents broken promises. A shattered heart correlates with the effects of broken promises.

Narcissism is a trait that is reflected really lucidly on Rose Mary. Rose Mary, while she never gets violent like Rex, seems to care about her own hobbies more than the well-being of the family, spending money on over 10 binders, and tons of art supplies. They also leave the door and windows open even after a pedophile gets in and tries to molest Jeannette, refusing to consider closing the house down at night. They spur the kids to do dangerous things like 'pervert hunting'. She thinks the world revolves around her; that she'll be a great artist. In the quote, Rose Mary demonstrates sorrow because she isn't going to New York as Jeannette is, telling her that she will not miss her but that travelling to New York has been an ambition to her. She's implying that it's more important for her to go to New York rather than her own child; she didn't even congratulate her nor support her, but was rather jealous. The image portrays the image of a narcissist: someone who loves himself the most and think they're the most important.

Jeannette has always been innocent. She loves her parents and mostly she loves her father. They've had a close relationship. As Jeannette matures, she starts seeing situations much clearer ; she stops looking up to her father as she used to do. She's able to deduce that all of the family's hardships are connected to with the lack of responsibility and motivation found in her parents. Jeannette, Lori and Brian are the parents to each other because Rose Mary and Rex didn't fulfill those roles in their lives: Rex steals the paychecks, destroys Oz, is never able to keep a job, stays limitless days and nights away from his family getting drunk and playing poker; Rose Mary is a narcissist who thinks of her kids are peripheral to her life, not providing for them, has more sympathy towards rats, canned food  and Hitler more than her own children.

Jeannette is able to comprehend life and starts becoming independent, working after graduating from a grandiose college: Barnard, and afterwards getting married. The image represents coming of age as gaining knowledge as a correlation to Jeannette's experience.