Taipei, officially Taipei City, serves as the capital and a special municipality of Taiwan. Located in Northern Taiwan, it is an enclave within New Taipei City, situated approximately 25 kilometres southwest of the northern port city of Keelung. Most of the urban area rests on the Taipei Basin, an ancient lakebed bounded by the valleys of the Keelung and Xindian rivers, which join to form the Tamsui River along the western border.