Fflewddur's Kingdom was a cantrev of the Northern Realms, ruled by Fflewddur Fflam. It was a tiny land, perhaps the size of a village and surrounding fields, in the eastern portion of the Northern Realms. Fflewddur found the place dreary and dull, so he chose to wander as an unofficial bard. However, he did once recall his "small drafty castle" with fondness, and admitted that he missed the place when he had been away too long.
The House of Fflam ruled the area for many generations. In The Chronicles of Prydain the reader never visits Fflewddur's realm, nor is it ever described to us, beyond the general character noted above. Nor do we learn its name, though given that no other character in the series has a proper surname, we may perhaps deduce that the name of Fflewddur's kingdom was also Fflam.
In Taran Wanderer, Fflewddur claimed that he must return to his kingdom because his subjects missed him. When a harp string snapped, the aspiring bard admitted he was anxious to see his people, and confided that they seemed to manage well during his many extended absences.
In The High King, Fflewddur parlayed his royal status into gathering a sizable force of warriors from among the Northern Realms. This army would go on to join the forces of the Sons of Don against the armies of Arawn Death-Lord.