How does Scout define the "caste system" of Macomb? What meaning is carried by the term "castle system"?

From the text:

There was indeed a caste system in Maycomb, but to my mind it worked this way: the older citizens, the present generation of people who had lived side by side for years and years, were utterly predictable to one another: they took for granted attitudes, character shadings, even gestures, as having been repeated in each generation and refined by time. Thus the dicta No Crawford Minds His Own Business, Every Third Merriweather Is Morbid, The Truth Is Not in the Delafields, All the Bufords Walk Like That, were simply guides to daily living: never take a check from a Delafield without a discreet call to the bank; Miss Maudie Atkinson’s shoulder stoops because she was a Buford; if Mrs. Grace Merriweather sips gin out of Lydia E. Pinkham bottles it’s nothing unusual—her mother did the same.

The caste system defines the small town's social hierarchy.