THE CAPITAL OF JAPAN, Tokyo is 1 of the world’s largest, most densely populated, and most influential cities, with a population of more than 12 million (2003). The Tokyo Metropolis (Tôkyô-to) has an area of 828 square mile (2,145 square kilometer) and for administration purposes consists of 23 wards, 26 cities, 1 county, 4 island administrative units, and 15 towns and villages. Combining the metropolis with suburban areas in neighboring prefectures of Saitama, Kanagawa, and Chiba creates an urban agglomeration known as Greater Tokyo, which constitutes the world’s largest such area, home to nearly 32 million, about a quarter of the population of Japan. The expanse of Greater Tokyo and its huge population makes the crowded average commute for workers in Tokyo more than 90 minutes each way. Tokyo sits at the confluence of several rivers, facing Tokyo Bay. None of these rivers, including the Sumida, Tone, and Edo, are navigable very far upstream. Conversely, none have been the source of significant flooding. These modest rivers and streams have necessitated hundreds of bridges in Tokyo, including the central Nihonbashi, from which all distances in Japan were traditionally measured. The eastern half of the central city constituted the old commercial downtown district (Shitamachi), while the western hills (Yamanote) were home to metroplolitan Tokyo’s more affluent residential areas. In the post- World War II period, large sections of Tokyo Bay were filled and new sections of city built upon what was once water. An extraordinarily complex web of public and private railway lines, subways, highways, and roads converge upon Tokyo, linking it to surrounding suburbs and the rest of the country. Nodes of rail transportation along the loop circling the city have developed into the major commercial districts of postwar Tokyo, notably Ikebukuro, Shinjuku, and Shibuya. The railway station at Shinjuku is the world’s busiest. Since Tokyo Bay was silt-clogged and shallow, the natural port city of Yokohama, about 25 mile (40 kilometer) to the south, grew as the region’s shipping center. The 2 cities are today a continuous industrial urban conurbation of tremendous concentration, although the district between Tokyo and Yokohama is separately incorporated as the municipality of Kawasaki. Tokyo emerged as a city relatively late in Japanese history, long after the cities of Osaka and Kyoto. It was only after the Tokugawa bakufu established its headquarters in what had been a sleepy fishing village in 1603 that Tokyo became a city. Soon Edo (as Tokyo was called until 1868) was the largest city in the land, and the first- or second-largest in the world, with a population exceeding 1 million by the 18th century. Edo was the shogun’s capital, home to the largest castle in the land, situated at the strategic point where the flat lands to the east began to rise westward. (Remnants of the walls and moats that protected the Edo castle from battles that never came can be found surrounding the Imperial Palace in the heart of today’s Tokyo.) Founder of the regime that bore his name, Tokugawa Ieyasu chose to make Edo his base. After Ieyasu’s appointment as shogun in 1603, he began a series of policies that made Tokyo a huge city and spawned more than 200 nascent urban areas around the country, the “castle towns.” (Today, there are 10 Japanese cities aside from Tokyo with a population of 1 million or more.) Edo, along with other Japanese castle towns, grew into cities as a result of the alternate-attendance system (sankin kôtai). Semi-independent local rulers (daimyô) were compelled to spend half their time in Edo, and half in their castle seats. Thus did Edo become a city, home not only to the shogun and his government, but to fortified mansions of the more than 200 lords (daimyô) who pledged loyalty to him. Because of this policy, about half of the population of Edo in the Tokugawa period (1600–1868) were samurai, much higher than in any of the castle towns, where merchants ostensibly serving the samurai class quickly outnumbered them.