It's got to be a Roman emporer or general, but it hinges on what one means by lost. Britain was lost by about 420 C. E. The Romans lost most of Gaul to the Huns under Atilla, but kept parts until a bit later. 'Riothamus' (probably Emrys Lydlaw/Ambrosius Aurelianus) was fighting in Gaul in about 460 and there were still some Romans around. This would best fit Valentinian III, but he does not seem to have marched with his troops. Reamoney may be Cremonea/ Cremona. The fact that this song in in English means it can't be nearly that old. From an English or Broadway musical?