If we are supposed to be in a medieval technology era, then coinage would really be the best method of currency, paper money wasn't viable in this world until 1694 and that required complex economics such as national debt. Plus going by paper, even if rare would be easy to duplicate as the masses would not know about the specifics of manufacture and the currency itself would most likely be drawn by hand, and forgers would have a field day making millions of fakes and causing economies to collapse from hyperinflation due to massive currency overloads.