Although the exact explanatory sentence is not fully detailed in the provided text chunks, the context provides the necessary clues: the extremely dangerous roads where 'one slip and everything is finished' and the hills where laborers break stones amidst flying clouds. By synthesizing these clues, we understand that the laborers work fearlessly in life-threatening conditions, which elevates their hard work to a king-like status, hence the phrase 'city of hardworking kings'.