Tokyo was originally known as Edo, which means "estuary".
Edo was first fortified by the Edo clan, in the late twelfth century. In 1457, Ōta Dōkan built Edo Castle. In 1590, Tokugawa Ieyasu made Edo his base and when he became shogun in 1603, the town became the center of his nationwide military government. During the subsequent Edo period, Edo grew into one of the largest cities in the world with a population topping one million by the 18th century.Tokyo became the de facto capital of Japan even while the emperor lived in Kyoto, the imperial capital.