We know the Silk Road was operating during Roman times. So Roman coins got as far as China.
https://www.britannica.com/money/topic/Silk-Road-trade-route
We know that China had trade relations with Japan.
Therefore it is not surprising that Roman coins diffused to Japan via China and probably India.
Had to click on the site of the odious NPR because I wanted to know the age of the castle. It was built in the Middle Ages.
Constantine the Great ruled, of course, in the 4th century AD, so perhaps, as someone suggested upthread, a coin collector lived in the castle. Or perhaps the coins were used for payment since gold in any form would be accepted for tender.
Exactly.