外堀の土塁 The Structure of the Outer Moat Ground 外堀土塁の構造 The Structure of the Base 牛込門から赤坂門までの江戸城外堀は、谷地形を利用し、広い堀幅の水面と、水面から高い土塁を持つ江戸城防御のための空間で構成されています。目の前に見える土手もその土塁の一部であり、2013（平成25）年から始まった石垣修復工事、その後のJR飯田橋駅西口駅舎等の工事に併せ調査が行われ、桝形石垣付近は江戸時代当時の規模を目安とした土塁の復元をしています。 外堀の土塁は、厚さ５～10cm程度ごとに土や粘土、砂など様々な種類の土を積み重ねて突き固められた版築と呼ばれる技法で作られています。地層断面をみると、土手の斜面方向とは逆に、水面の方に向かって地層が上がっています。これは、土砂崩れを防ぐためと考えられています。土塁の道路端には土留めの石垣が築かれていましたが、現在では土手に沿って通る道路の端に確認できる通り、コンクリートの小型擁壁に代わっています。
The section of Edo Castle’s outer moat between Ushigome and Akasaka Gates was constructed by utilizing the valley topography, which allowed for the development of a wide canal surface and tall earthwork fortifications. The embankment visible here formed a portion of the area’s earthen fortifications. In 2013, a series of excavations were carried out in conjunction with a project to repair the stone walls and redevelop the western side of JR Iidabashi Station. The project enabled the reconstruction of earthen fortifications on the same scale as those that existed in the Edo era. As a result, contemporary visitors to the site can experience the grand scale of the historical fortifications that existed in the area. The outer moat’s fortifications were constructed using a rammed-earth technique in which 5-to-10-centimeter layers of earth and clay were stacked on top of one another and tamped down. An examination of the fortifications geological profile reveals that the layers of earth and clay rise towards the towards the moat’s surface and away from the embankment slope, It is likely that this construction method was utilized in order to prevent landslides. Earth-reinforced stone walls were constructed along the roadside lining the earthen fortifications. However, as can be determined from the roadside currently located parallel to the embankment, the original earth-reinforced walls have been replaced by small-scale concrete retaining walls.
外堀土塁の水際部分（発見された水際石垣） Waterside Part of The Outer Moat Ground (Discovered Waterside Stone Wall 2015（平成27）年に行われた発掘調査の結果、外堀の水際には、土手が崩れないようにするための低い石垣が作られていたことが判明しました。発見された場所は、牛込橋から140m程度市谷方向に進む地点の線路の間で、石垣の基礎部分と考えられる 1〜2段の石積みが、外堀で初めて確認されました。この石垣は、明治期に甲武鉄道が敷設された際に、上段部分が撤去され、基礎部分はそのまま埋められたものと考えられます。
An archaeological excavation carried out in 2015 revealed the existence of low stone walls along the moat’s edge. They were likely constructed in order to prevent the embankment from collapsing into the moat. The site where the walls were discovered is located in a section of rail track approximately 140 meters away from Ushigome-bashi Bridge in the direction of Ichigaya. At that site, excavators found one- and two-tier layers of stone, which are thought to have comprised the stone walls’ structural foundation. When the Kōbu Railway was constructed in the Meiji period, it is thought that the upper portion of the stone walls were removed and the foundation was buried.
外堀土塁の植栽 Planting of The Outer Moat Earth Fence 1636（寛永13）年、外堀土塁が完成すると、幕府は翌年、堀方七組の東国大名に命じて、土塁上部から2.7m程下がった位置に、一間（約1.8m）間隔で大きめの松杉を、その内側には小さめの苗木を、二筋にわたって植樹させました。植木奉行（のち普請奉行）の管理下に置かれた牛込土橋～筋違橋の土手と堀は、近辺に屋敷を拝領する武家に割振り、植木の手入れと草刈りを担当させました。幕末に日本を訪れたオイレンブルクの『日本遠征記』によれば、「城壁天端の平坦部とその内側にはモミなどの針葉がぎっしりと並び、水面には何千という野鴨が住み着いている」と記されています。なお、真田濠（現上智大学グラウンド）の土塁上には、往時の姿を思い浮かべることができるような松の植生がみられます。
The outer moat’s earthenworks were completed in 1636. The following year, the Tokugawa shogunate order the group of eastern Japanese domainal lords previously mobilized to dig portions of the outer moat to plant a row of pine and cedar trees approximately 2.7 meters below the upper portion of those fortifications. The trees were to be planted 1.8 meters apart. In addition, they planted an inner row of smaller saplings parallel with the pine and cedar trees. This project was carried out under the direction of the Governor of Landscaping (later the Governor of Construction) and the maintenance of the newly planted fauna was entrusted to the warrior houses occupying estates along the outer moat and embankment between Ushigome-bashi Bridge and Kuichigai Gate. During the late-Edo period, the area was visited by Prussian diplomat Count Friedrich Albrecht zu Eulenburg. In his Record of the Eulenburg Expedition to Japan, he wrote, “Thick rows of fir trees and other conifers line the flat section crowning the castle walls and the walls’ interior section, and thousands of wild ducks inhabit the moats surface.” The rows of pine trees lining the embankment alongside Sanada-bori Moat (present-day Jōchi University Field) provide visitors with a sense of what the outer moat area was like in the Edo period.
In 1636, Iemitsu, the third Tokugawa shogun, commanded domanial lords from around the country to construct an outer moat around Edo Castle. The following year, two rows of pine saplings were planted on the grass-covered embankment lining the outer moat. The outer row was comprised of large saplings, which were planted 2.7 meters from the embankment edge at intervals of 1.8 meters. The inner row consisted of an alternating series of large and small saplings arranged in a similar pattern. These rows supported the embarkment and shielded the castle. The portion of the embankment extending east from Ushigomemon Gate can be divided into two sections: one located along the outer moat and the other located inside the Gate. Photographic evidence indicates that the section lining the outer moat was a gentle earthen bank. The section inside the Gate is depicted in Views of Edo as a grass-covered landscape dotted with pine trees. During the Meiji period (1868-1912), the construction of wider roads and rail lines reduced the size of the embankment.
Underground, however, portions of the outer moat’s embankment, which constructed during the Tokugawa period using the ancient rammed earth (hanchiku) method, remain intact. This display recreates the earthen bank and rows of pines， which were located to the east Ushigomemon Gate’s box-shaped stone walls.
江戸城外堀 The History of Edo Castle Outer Moat 江戸城は、本丸・二の丸・三の丸・西の丸・北の丸・吹上からなる内郭を内堀が囲み、その表門が大手門でした。外堀は、雉子橋門から時計回りに、一橋門、神田橋門、常盤橋門など諸門をめぐり、呉服橋門から虎ノ門、溜池から四谷門、市谷門、牛込門を経て、現在の神田川に入り、小石川門から浅草門で、隅田川に至る堀でした。外堀工事は、1606（慶長11）年に雉子橋から溜池までの堀を構築後、1618（元和４）年に駿河台が掘削されて平川（現日本橋川）の流路に付け替えられ、神田川が誕生しました。この工事で、平川は堀留橋で締め切られ、独立した堀となりました。 1636（寛永13）年には、天下普請で外堀が構築され、江戸の総構が完成します。この工事は、雉子橋から虎ノ門に至る外堀の総石垣化と枡形築造を前田・細川・池田・黒田家など西国外様大名（石垣方六組）、牛込土橋から赤坂土橋にかけての外堀掘削と土塁の構築を東国大名（堀方七組）が行いました。 その後も幕府は、外堀を維持するために大名の手伝普請による堀さらいをしました。牛込～市谷間の堀は、市谷～四谷間より水位が下がり、土砂が堆積し、蓮が繁ったため、普請奉行の管理下で頻繁にさらいが行われました。また、町人にも堀にゴミを捨てないよう町触も出され、外堀の維持・管理が行われました。
Edo Castle’s inner hull was comprised of six citadels: the main, second, third, western, northern, and fukiage citadels. The entire inner hull was encircled by an internal moat and Ōte-mon Gate served as its main entrance. The Castle’s outer moat originated at Kijibashi-mon Gate and passed, in clockwise direction, through Hitotsubashi-mon, Kandabashi-mon, Tokiwa-bashi-mon, Gofukubashi-mon, and Torano-mon Gates. It then extended from Castle reservoir through Yotsuya-mon, Ichigaya-mon, and Ushigome-mon Gates before ultimately flowing into the present-day Kanda River. From there. it served as a canal, which passed through Koishikawa-mon and Asakusa-mon Gates and ultimately converged with the Sumida River. The outer moat’s development began in 1606 with the construction of a canal from Kiji-bashi Bridge to the Castle reservoir. The second stage in its evolution came in 1618, when portions of the Kanda Plateau were removed and the canal was redirected towards the Hira River. That process gave birth to the Kanda River and transformed the Hira River, which was closed off at Horidome-bashi Bridge, into an independent canal. The outer moat was finally completed in 1636, when feudal lords from eastern and western Japan were mobilized to construct its remaining portions. Specifically, western domainal lords, including the Maeda, Hosokawa, Ikeda, and Kuroda Houses, were ordered to supervise the construction of stone walls and the square masugata enclosures used to protect the castle gates along the section of outer moat between Kiji-bashi Bridge and Torano-mon Gate. In constrast, domainal lords from eastern Japan were order to dig a canal between Ushigome-bashi and Akasaka-bashi Bridges and construct earthen fortifications. In order to maintain the outer moat, the Tokugawa shogunate mobilized domainal lords from around the country to dredge it. In particular, the section of the moat between Ushigome and Ichigaya required frequent dredging because the water level often receded, resulting in the accumulation of silt on the moat floor and the development of water lilies on its surface. Such dredging projects were carried out under the supervision of the shogunate’s Governor of Construction. In addition, the city authori-ties attempted to keep the outer moat free of debris by issuing official proclamations banning the residents of commoner neigh-borhoods from disposing of garbage in the moat.
赤坂門：長崎大学附属図書館蔵
Akasaka-mon Gate: “Photo Tokyo Past and Present” Hiroshi Nozawa
喰違：『絵本江戸土産』国立国会図書館蔵
Kuichigai-mon Gate: “Edo Souvenir in Pictures” National Diet Library
そのほか：『旧江戸城写真帖』東京国立博物館蔵
Others: “Old Edo Castle Photo Collection” Tokyo National Museum
