Because there had been a range of hills from the Akasaka-yama hill (present-day Sakurazaka 2 chome), which is about 30 metres (98 ft) above sea level, to the hill which was to become the site of the "hon-maru", an area to the south of the castle was excavated to form a moat. The hills to the south of the moat were also excavated and altered to become gently sloping hills. The top section of a hillock to the northwest of the inner castle was also truncated because it was higher than the "hon-maru". The hillock was then used as the site for a retirement residence for Kuroda Yoshitaka.