What kind of terrain is called "Luocheng"

① Luocheng, a big city outside the city. "History of the North, Biography of Women, Wei Rencheng, the imperial concubine of the country": "The thief handsome Jiang Qingzhen rebelled against the party and attacked Luocheng." "Zi Zhi Tong Jian Tang Yizong Xian Tong Nine Years": "When he did not move, Luo Cheng and Yan Zeng surrendered to Zicheng." Hu Sansheng's note: "Luocheng is a big city outside; Zicheng, a small town in the city. "

(2) Luocheng in geomantic omen, also known as Yuan Ju, means that when Long Mai falls from the peak to Pingyang, the other branches will be wrapped around the city wall, enclosing the real dragon, so as not to make the flourishing gas leak out. Luocheng and Yuanju are both figurative words. Because of their dense entanglement, they are like a snare, hence the name Luocheng.