Tokyo is the capital of Japan. Population-wise, it is one of the largest cities in the world. Tokyo forms a special administrative district called the Tokyo Metropolis, which comprises the 23 wards, the Tama Area that includes 26 suburban cities, and 9 islands. The center of administration, economic activity, industry, culture, and so on, Tokyo is the hub of Japan. The 23 wards can be roughly divided into an upper (Yamanote) and a lower (Shitamachi) area. Tokyo developed around the Imperial Palace, which used to be the largest castle in Japan. An extensive network of urban trains, subways, and buses serves all parts of the city. The Chuo-sen, or Central Line, the Yamanote-sen, or Loop Line, of Japan Railways, as well as a number of subway lines are the basic transportation arteries in the Tokyo Metropolitan area.