Early pictorial map of Edo Castle unearthed in Matsuehttps://t.co/RjveVegYBr pic.twitter.com/SuoskR1fDQ — The Mainichi (@themainichi) February 9, 2017

The Edo Castle in Tokyo—now part of the city’s Imperial Palace—was once much grander than the buildings and walls that still exist today. That’s because it was originally designed as a home for the country’s shoguns, the line of dictators appointed by the Emperor who ruled Japan for centuries until the Meiji Restoration in 1868.

The castle also changed over the years after it was originally constructed in 1457, to account for the changing tastes of its residents, the constant threat of attack, and its own occasional destruction.

Maps of the castle have been recovered in the past, but there are many years for which historians simply don’t know how it might have looked, as other maps have been lost.

Edo Castle photographed circa 1870. Felice Beato/Public Domain

But recently another map of the castle—one of the earliest found—was discovered, filling in some of the blanks and shedding light on how it looked in the early 1600s, according to The Mainichi.

The new map reveals that the castle appears to have been fortified by shogun Tokugawa Ieyasu in the years before the Siege of Osaka, a major battle which took place in 1614 and 1615, when Ieyasu and his armies defeated a rogue clan.

The newly discovered map also shows that in the years before battle, Ieyasu was already beginning to prepare.

“The map is evidence that Ieyasu had a major battle in mind,” one specialist told The Mainichi. “Ieyasu built what could be the strongest castle of the time.”

Among the fortifications: a “quintuple-entrance structure,” according to the Japan News, in addition to a series of corridors that connected the castle’s keeps.

The new map was recovered some 400 miles to the west of Tokyo, by officials conducting a survey of collections at the Matsue History Museum. Officials did not say how it ended up there.