What's the message from the poem of Han Chuo, Yangzhou magistrate?

The Letter to Han Chuo, the Magistrate of Yangzhou, was written by Du Mu, a poet in the Tang Dynasty, after he left Yangzhou, in memory of Han Chuo, an old fellow judge. This poem deliberately depicts that Yangzhou is still lush in late autumn, and the moonlight music on the Twenty-four Bridges is still melodious, mocking the leisure of friends' lives and expressing the author's deep nostalgia for Yangzhou's past lives. The whole poem is beautiful, delicate and handsome, and full of fun.