Make a sentence with caste.
I assume you mean make a statement about the story in conjunction with the idea of caste. Caste is the Indian social system that separates people into separate categories, from high to low, and asserts that movement between the categories can not exist.
Siddhartha's journey proves that the caste system is counterproductive to finding inner peace.
Siddhartha should be at peace because he belongs to a high caste, but he is unhappy to the point of being suicidal. It is when he abandons the social structure that he begins to find peace.