In 1590, when Tokugawa Ieyasu and his subordinates moved into the Kanto region, the hill on which Edo Castle stood was surrounded by marshes and mud flats. A broad inlet known as the Hibiya irie (Hibiya inlet) lay right at the base of this hill. It served as a good protective defence when the castle was built, but it was too shallow to accomodate ships. Between the Hibiya irie and the mouth of the Sumida river was a low, sandy peninsula. The island was muddy and often submerged by high tides, so it could not be used for either farming or to build houses. The Koishi River -- the main water source for Edo, crossed this peninsula and emptied into the Sumida river right near its mouth.
Although Ieyasu chose Edo as his new capital because of the marvelous defensive position of the castle, it was clear right away that the marshes and mud flats were more of a nuisance than a benefit. One of the first things he did was to start reclaiming the land from the bay and turning it into a town. Ieyasu's soldiers entirely leveled Kanda Hill, which had been on the opposite bank of the Ishikawa river from Edo Castle. The dirt and rock taken from Kanda Hill was used to fill in the Hibiya irie and raise the height of the sandy peninsula. Several canals were dug through the area to facilitate transportation. By 1601, most of the area had been converted to dry land, and a town was beginning to spring up on the flat plain beneath the castle walls.