Archaeologists working at the ruins of an historic castle in Japan’s Okinawa Prefecture turned up a surprising discovery recently, when they unearthed four ancient Roman coins dated to 300–400 CE—the first time such artifacts have been discovered anywhere in Japan.

Archaeologists, who have been excavating the site of Katsuren Castle in Japan’s southernmost prefecture since 2013, have also found six bronze coins that they believe date to the Ottoman Empire in the late 17th century, however the city of Uruma’s Board of Education, which announced the remarkable discovery on Monday, said the Roman coins appeared to be far older.

“I’d come to analyze artifacts like Japanese samurai armor that had been found there when I spotted the coins,” said visiting researcher Toshio Tsukamoto. “I’d been on excavation sites in Egypt and Italy and had seen a lot of Roman coins before, so I recognized them immediately.” Tsukamoto speculated that the coins had reached Japan after passing along trade routes linking Asia with the West. (CNN)

Subsequent X-ray analysis of the copper coins, which are 1.6–2 centimeters in diameter, showed that some were enscribed with Roman letters and what appeared to be images of the Roman emperor Constantine I and a soldier wielding a spear.

“I couldn't believe they'd found coins from the Roman Empire in Kasturen Castle. I thought that they were replicas that had been dropped there by tourists,” said archaeologist Hiroki Miyagi of Okinawa International University. “But after washing them in water I realized they were much older. I was really shocked.” (CNN, Rappler)

Archeologists are uncertain how the coins found there way to the castle, believed to have been established as early as the 12th century and to have been abandoned at some point in the 15th century. Although it was once home to a feudal lord whose wealth was linked to regional trade, it is unlikely that he had any interaction with Europe. 

Masaki Yokou, a spokesperson for the Board of Education, conceded that while Okinawa’s trade with China and Southeast Asia was believed to have been thriving during the period, it would be hard to determine where exactly these coins came from. “We don’t think that there is a direct link between the Roman Empire and Katsuren Castle,” he said. “But the discovery confirms how this region had trade relations with the rest of Asia.” (CNN)

Other artifacts unearthed at the site include Japanese ceramics and other objects used by the castle’s inhabitants, as well as Chinese coins and ceramics that would have been acquired through trade with China. “The Chinese ceramics and coins that we found date back at least 600–700 years and we’d like to analyze those objects in tandem with these coins to work out how the coins may have ended up here,” said Miyagi. “East Asian merchants in the 14th and 15th centuries mainly used Chinese currency—a round coin with a square hole in the middle—so it is unlikely that the Western coins were used as a means of currency. I believe they probably got the coins in Southeast Asia or China.” (CNN, Rappler)

The coins will be on public display at Uruma City Yonagusuku Historical Museum in central Okinawa until 25 November.

Katsuren Castle was recognized as a Designated Historical Monument by the Japanese Agency for Cultural Affairs in 1972, and in 2000, was listed as one of nine UNESCO World Heritage Sites that make up the Gusuku Sites and Related Properties of the Kingdom of Ryukyu.

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Ancient Roman coins found in ruined Japanese castle (CNN)
Ancient Roman coins unearthed from castle ruins in Okinawa (The Japan Times)
Ancient Roman coins found buried under ruins of Japanese castle leave archaeologists baffled (Independent)
Ancient Roman coins unearthed from castle ruins in Okinawa (The Mainichi)
Ancient Roman coins unearthed at Japan castle (Rappler)