The given excerpt is taken from the part of the novel where Casey, a twelve year old girl and the main character of the story, is sent to live with her grandmother in Chinatown. For Casey, Chinatown represents a journey of exploration and discovery, at once daunting and exciting, and the author's word choices and use of imagery reflect these mixed emotions.
The metaphorical image of breaking through "an invisible wall into another world" emphasizes how completely different Chinatown is from what Casey is used to. The image of breaking through a solid "wall" emphasizes just how entirely separate this place is from what Casey has known previously. The idea that Chinatown is "another world" suggests that, to Casey, it is as unusual and alien as a different planet might be. This difference is also emphasized by the author's use of comparative adjectives such as "brighter" and "lighter," used to describe the air of Chinatown relative to the air that Casey has been used to breathing elsewhere.
The author also conveys the impression that there is much for Casey to discover in Chinatown. He uses prepositional phrases such as "on the north side," "on the west," and "on the other two sides" to suggest that Casey is completely surrounded by new places. The fact that she is completely surrounded also suggests that this journey of exploration and discovery is, at this point, somewhat overwhelming. This impression is also suggested by words which connote the huge scale of the buildings, such as "mansions" and "tall skyscrapers." This impression of an overwhelming somewhat daunting experience is emphasized further when we remember that Casey is just twelve years old.