丸ﾉ内
Marunouchi  (Inside the Circle; more at “Inside the Moat”)

The derivation of today’s place name is pretty famous, even among foreigners. It’s so famous that even the English version of Wikipedia got it right.

The area was part of the old Hibiya Inlet which fell under the influence of Chiyoda Castle (Edo Castle) and merged into the castle and Yamanote district.

丸 maru (literally circle) was used to refer to 城内 jōnai (inner) castle grounds*  Buildings inside the moat (usually ring shaped) had names like 本丸 honmaru (main citadel), 二ﾉ丸 ninomaru (secondary citadel), 三ﾉ丸 sannomaru (tertiary citadel), etc**.  The shōgun (or lord) lived in the honmaru (first citadel) as it was surrounded by all the other maru. By this system of naming, it’s easy to see how 丸ﾉ内 Marunouchi (inside the circle) could refer to buildings inside the moat (scil; the circle).

Over the years Marunouchi has been written a few ways,
the last two being preferred these days:

丸内
丸之内
丸ノ内
丸の内

丸ﾉ内 Marunouchi was colloquially referred to as 大名小路 daimyō kōji daimyō alley***. It was located on the castle grounds, between the outer moat and inner moat on a small man-made island with its own system of gates and mitsuke’s that made it more or less indistinguishable from the castle proper.

Today, nothing remains of the castle in the area so it’s hard to imagine that this was part of the outermost ring of Japan’s largest castle. However, in the Edo Period, the area was extremely important to the shōgunate. At one point there were 24 daimyō residences here. A list of noble names makes up its residences: Ii, Honda, Sakai, Sakakibara, and Hitotsubashi & Matsudaira**** – to name a few. The daimyō who lived here were mostly hereditary vassals of the Tokugawa shōgun family. That is to say, they were the elite of the elite and the closest to the shōgun. That’s why it was so important that they have palaces within the castle walls.

In the Meiji Era (here we go again), the daimyō were all kicked out and daimyō lands were all confiscated by the new Imperial government.  It seems that the daimyō residences were all knocked down and the area became a training ground for the Imperial Army. A lot of the land was eventually purchased by the forerunner of the Mitsubishi Group and still remains their real estate (can anyone say good business deal?).

Since the area was just grass and roads and barracks, when Mitsubishi knocked down the few remaining the structures the area was basically a field. In fact from 1890 to about 1910 only the Tōkyō Municipal Government Building and a few Mitsubishi buildings stood in 三菱ヶ原 Mitsubishigahara Mitsubishi Fields. Mitsubishi used the area to build up a central business district and the completion of Tōkyō Station in 1914 definitely sped up that process.

Much of the outer moat was filled in after WWII during the massive building efforts that eventually propelled Japan into its legendary “bubble economy.” In fact, if you look back at my articles on Nihonbashi, Kyōbashi, Hatchōbori, Akasaka and Akasaka-mitsuke, you’ll see that most of those areas are all on solid ground now with no moats or canals in sight. The ability to walk (without crossing a bridge from Marunouchi to Yaesu and then to Edomachi (Nihonbashi) would have been shocking to a resident of Edo (read those other links to figure out why)

Today the old Daimyō Alley includes:
・ Tōkyō Station (about 12 residences stood here)
・ Hitotsubashi Junction
・ Ōtemachi (about 10 residences stood here)
・ Hibiya Station (but not Hibiya Park; about 4 residences)
・ Yūrakuchō (about 8 residences)
・ The Kubizuka of Taira no Masakado (one of the most haunted spots in Tōkyō;
daimyō families whose residences maintained the kubizuka were the Doi and Sakai)
・ The Imperial Palace Park (Kōkyo Gaien) (formerly 9 major residences occupied this space;
it could be argued that this was part of the castle and not daimyō alley)

丸 maru can also be referred to as 郭 or 曲輪, both read as kuruwa.
Another set of synonyms to 本丸 and the like existed: ―ﾉ郭, 一ﾉ曲輪 ichinokuruwa (first citadel), 二ﾉ郭, 二ﾉ曲輪 ninokuruwa (second citadel), etc.
 The area called “Daimyō Alley” took its name from main north/south running street. You can see it on the map. The nickname came to refer the area in general. That said, this was the only long, straight street. Most streets around the castle were intentionally maze-like.
**** Matsudaira, as you may already know, was the Tokugawa family before Ieyasu took the Tokugawa name. For reasons I don’t want to get into now, if you see the kanji 平 (taira/daira) in an old Japanese name, you can assume it’s a noble name. Hitotsubashi is also a collateral family of the Tokugawa.  Fans of the Sengoku era will know why I listed the other 4 names.

PS: for all info about Japanese castles, please check out: Jcastle.info.
PPS: for all info about samurai, please check out: Samurai-Archives.
PPPS: for a fucking awesome collection of pictures of Tokyo Station, please check out: Japan Web Magazine.