In 1524, the settlement called Edo (江戸) was a minor place — a fishing village and small trading post at the mouth of rivers flowing into a bay on the eastern edge of the Kantō (関東) plain. The name itself is believed to come from "e-do" (江戸), meaning "bay entrance" or "estuary door." The castle that controlled this bay entrance was a modest earthwork fortification on the Musashino (武蔵野) plateau above the water, commanding the roads north and west. Built in 1457 by Ōta Dōkan (太田道灌), a brilliant military architect who understood the site's strategic value, it had passed into the hands of the Uesugi (上杉) clan. By 1524, Uesugi Tomooki (上杉朝興) held it.

Hōjō Ujitsuna (北条氏綱) was 37 years old in 1524, the son of Hōjō Sōun (北条早雲) — the remarkable self-made warlord who had founded the Later Hōjō clan through a combination of brilliant strategy and patient opportunism. Ujitsuna was continuing his father's campaign to bring the entire Kantō plain under Hōjō control, domain by domain. Edo Castle was a prize: whoever held it controlled the sea access and the road network of eastern Kantō. In 1524, Ujitsuna marched north with a force of over 10,000 men, crossed the Arakawa (荒川) River, and moved toward the Uesugi position.

The engagement at the Battle of Takanawahara (高輪原の戦い, Takanawahara no tatakai), near present-day Meguro, demonstrated the tactical cunning the Hōjō had inherited from Sōun. Rather than attacking Tomooki's forces directly, Ujitsuna maneuvered his troops around the Uesugi position and struck from the rear. Tomooki, outflanked and beaten, retreated toward the safety of Edo Castle. He had lost the battle but expected to hold the walls.

He arrived to find the gates already open. Ōta Suketaka (太田資高), the garrison commander, had betrayed him. The Ōta family name is significant: it was Ōta Dōkan (太田道灌) — almost certainly a relative — who had built Edo Castle 67 years earlier. The family that had created the castle now handed it to the enemy. Tomooki had nowhere left to go. Edo Castle fell to the Hōjō without a siege, taken by treachery from within rather than force from without.

The Hōjō seizure of Edo Castle in 1524 began seventeen years of intermittent war between the Hōjō and the Uesugi for dominance of the Kantō. Ujitsuna and his successors proved as capable as Sōun had been, and the Later Hōjō held Edo Castle and the surrounding region for sixty-six years. In 1590, Toyotomi Hideyoshi (豊臣秀吉) launched a massive campaign to unify Japan under his authority. The Hōjō, who had refused to submit, were besieged at Odawara (小田原) and forced to surrender. They were driven from Edo Castle, and the domain was redistributed.

The domain of Edo was awarded to Tokugawa Ieyasu (徳川家康) as part of the post-campaign settlement. Ieyasu looked at the modest earthwork on the bay and understood what Ōta Dōkan had understood in 1457 and what the Hōjō had confirmed in 1524: the site was irreplaceable. He built. Over the following decades, Edo Castle was expanded into the greatest castle complex in Japan — and eventually all of Asia — with concentric moats, massive stone walls, a five-story donjon (天守閣, tenshukaku), and the infrastructure of a capital city spreading in every direction. When Ieyasu became shogun in 1603, Edo became the political capital of Japan.

The Edo period ended in 1868 with the Meiji Restoration (明治維新, Meiji Ishin). The new imperial government took possession of Edo Castle and renamed both the city and the castle grounds. The city became Tokyo (東京, "Eastern Capital"). Edo Castle became the Imperial Palace (皇居, Kōkyo). The modest earthwork that Ōta Dōkan built, the Hōjō seized, the Uesugi lost, and Ieyasu transformed has been the most important political site in Japan continuously for more than five centuries. The fishing village at the bay entrance is now a city of over thirteen million people.