Katsuren Castle was known to have been the focal point of trading partnerships with China and other Asian countries, but ties to Europe were not evident until the recovery of the coins.
The ruins of the castle were registered in 2000 on the World Heritage list as part of the Gusuku Sites and Related Properties of the Kingdom of Ryukyu.
Ottoman coins and other relics were also found among the ruins (Urama Board of Education)
Education spokesman Masaki Yokou told CNN: "It is a strange and interesting find. We don't think that there is a direct link between the Roman Empire and Katsuren Castle, but the discovery confirms how this region had trade relations with the rest of Asia."
The coins will be analysed further and displayed at Uruma City museum on Okinawa until the end of November.