The drawing was apparently produced between 1607 and 1609 just after the castle's construction in today's Tokyo was ordered by Tokugawa Ieyasu (1542-1616).
The finding puts the drawing titled "Edo Hajimezu," (Map of early Edo) which is kept at the Matsue History Museum, in the same age bracket as the oldest known drawing of the fortress.
A recent examination of the illustration also shows that it was being constructed with a mighty clash between warring clans in mind.
“The details of the construction show that the Tokugawa family were taking all possible measures to ensure their preparation for another battle against the Toyotomi clan,” said Yoshihiro Senda, professor of castle archaeology at Nara University, and who studied the drawing.
Ieyasu, the founder of the Tokugawa Shogunate (1603-1867), consolidated his grip on power by defeating the Toyotomi clan in the 1614-1615 siege of Osaka Castle.
The drawing originates from around the same time as the "Keicho Edo Ezu" (Keicho Edo map) kept at the Tokyo Metropolitan Central Library, which is the oldest known map depicting the castle.
Both portray the central part of the castle, but the "Edo Hajimezu" is more informative about the fortress, according to experts.
“It provides clues about the detailed arrangement of the central area of the castle, with full descriptions of stone walls and mounds,” said one of the historians who scrutinized the ground plan.
The existence of the drawing, 27.6 centimeters by 40 cm, at the museum was known, but the full details not revealed until now.
The illustration's age was narrowed down for the first time during the recent study.
The drawing shows the Tsumenomaru quarter in the castle’s main compound, the complex structure of the castle walls and also what are believed to be the names of occupants in residences around the site.
Senda said it is difficult to make a clear distinction between stone walls and buildings in "Keicho Edo Ezu" because there is large distortion, but "Edo Hajimezu" confirms large and small keeps huddled together, like at Himeji Castle in Hyogo Prefecture.
He added that it is also evident that the walls and gates in the south of the castle were arranged in a zigzag fashion to prevent the enemy from advancing forward in a straight line. This kind of defensive design, called "Gorenzoku Soto Masugata" (five consecutive rectangular spaces), was also built at Kumamoto Castle in Kyushu.
Edo Castle was repeatedly revamped later, making it difficult to figure out the initial design of the castle under Ieyasu until the recent discovery. Today, there are no remains of keeps standing close to each other or of the consecutive rectangular spaces.