The portion on the west side that is narrower in length along the north-south line was added during Iemitsu TOKUGAWA's renovations in the Kanei period, so the eastern half was all there was at the time Ieyasu built it.
南北の幅が狭くなっている西側部分が徳川家光の時代に行われた寛永の大改修によって拡張された部分で、家康による創建時は現在の東側半分のみであった。