Rohestoc, Rorestok, Rostock, Rostok
Lot and his sons were on their way from Logres back to Arestuel in Scotland when they met 7.000 Sesnes leading 700 prisoners on the plains of Roestoc. They battled the Sesnes (Saxons), at which time Gawaine won his horse Gringolet from King Clarions, though he did not kill Clarions.
Since it had both plains and a ruling lady, we are probably safe in assuming Roestoc was a subkingdom. In the sub-county of North Riding, Yorkshire, are the north York moors, which might have been the plains of Roestoc.
Lady of Roestoc | The Legend of King Arthur