San Diego is located in the southwest corner of the United States, on the shores of San Diego Bay on the southern tip of California, borders Mexico on the south, and is the seventh largest city in the United States. The Old Town of San Diego is the birthplace of San Diego and the first colony of Spain in California. Old Town San Diego is a beautiful and fun-filled historical city full of Mexican flair. It is the birthplace of California and still retains the quaint architecture and quiet life of the 19th century.