Moreover, the third shogun, Iemitsu TOKUGAWA decided to reinforce the castle's western exterior enclosure, which was left untouched, and built an outer moat using the reservoirs and streams that coursed through Akasaka, Yotsuya, Ichigaya to Ushigome before flowing into Kanda-gawa River.
更に3代将軍徳川家光はこれまで手薄で残されてきた城の西部外郭を固めることにし、溜池や神田川に注ぎ込む小川の谷筋を利用して溜池から赤坂、四ッ谷、市ヶ谷を経て牛込に至り、神田川に接する外堀を造らせた。