San Diego (/ˌsæn diˈeɪɡoʊ/ SAN dee-AY-goh, Spanish: [san ˈdjeɣo]; Spanish for 'Saint Didacus') is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United States and the seat of San Diego County, the fifth most populous county in the United States, with 3,338,330 estimated residents as of 2019. The city is known for its mild year-round climate, natural deep-water harbor, extensive beaches and parks, long association with the United States Navy, and recent emergence as a healthcare and biotechnology development center. San Diego is the second largest city in the state of California, after Los Angeles.
Jiayuguan, known as "the first male pass in the world", is located in the middle of the narrowest valley 5 kilometers west of Jiayuguan City, Gansu Province. the city walls on both sides of the pass cross the Gobi Desert, the suspended Great Wall of Montenegro in the north and the first pier in the south. it is the westernmost pass of the Ming Great Wall. it was once known as Hexi throat in history, because of its dangerous topography and majestic architecture, it is known as the chain key of the border. Jiayuguan is the traffic fortress of the ancient "Silk Road" and one of the three wonders of the Great Wall of China (Shanhaiguan in the east, Zhenbeitai in the middle and Jiayuguan in the west). Jiayuguan was built in the fifth year of Hongwu of the Ming Dynasty (1372). It is composed of inner city, outer city, Luocheng, Wengcheng, trench and the north and south wings of the Great Wall, with a total length of about 60 kilometers. The platform, pier and fortress of the Great Wall are dotted with three lines of defense, namely, the inner city, the outer city and the moat.