The first sentence of "Nanxiangzi Dengjingkou Gu Bei Ting Huai" is "Where to see China? What do you mean by "seeing Feng patronizing the North Building"? What role does it play in structure?

"Where to see China? Eyeful Feng Guang Gubei Building. " At this time, the Southern Song Dynasty was separated from Jin by the Huaihe River, and Xin Qiji stood on the North Drum Tower on the bank of the Yangtze River, looking at the occupied area of Jinbing in the north of the Yangtze River, which had the feeling that the scenery was gone and the mountains and rivers were discolored. The implication is that the Central Plains is no longer mine. So standing on the north tower, the poet can't help feeling the rise and fall of the ages, which naturally leads to the following.