Ancient Roman coins have been discovered in the ruins of a castle in Okinawa, Japan's southernmost prefecture. It was the first time that such artifact was discovered in Japan.

Four copper coins that likely date back to the Roman Empire were unearthed in the ruins of the Katsuren Castle, a UNESCO World Heritage site. The castle was built sometime between the 13th and 14 century, but it was abandoned 200 years later. Archaeologists have been excavating this site since 2013.

The castle used to be the residence of a feudal lord whose wealth was linked to regional trade. It is not, however, known if he engaged in business with traders from Europe.

The copper coins measure 1.6 to 2 cm (0.63 to 0.79 inch) in diameter, but the designs and patterns on both their sides are no longer clear because of abrasion. X-ray analysis, however, revealed that the coins bear the image of Constantine the Great and a soldier holding a spear.

The find came as a surprise. Archeologist Hiroyuki Miyagi, who works at the Okinawa International University, thought it's a hoax when he first heard that ancient Roman coins have been discovered in Okinawa.

"I thought that they were replicas that had been dropped there by tourists," Miyagi said.

The Uruma City's Board of Education, which carried out the excavation, said that the find suggests of the region's wide trade relationships.

Okinawa's trading relationships with China and other Asian countries were thriving between the 14th and 15th century, and the discovery points at a possible link between Okinawa and the Western world, said Uruma City's Board of Education spokesperson Masaki Yokou.

Toshio Tsukamoto from the Gangoji temple cultural properties department, who spotted the coins when he traveled to the castle, think that the Roman coins ended up in Japan after it passed through different trade routes that linked the West to Asia.

"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," said Miyagi.

Other artifacts that were discovered at the excavation site included a coin from the 17th century Ottoman empire and five other round metallic items that appear like coins.

The Ottoman coin has inscriptions revealing it dates back to 1687. The Roman coins were older and are estimated to date back to 300 to 400 AD.