Rohestoc, Rorestok, Rostock, Rostok
Lot and his sons were on their way from Logres back to Arestuel in Scotland when they met 7000 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.
Roestoc, Lady of
She seems to have been Roestoc's ruler. She fell in love with Gawaine and gave him a girdle and locket when he defeated Sir Segurades for her. Gawaine subsequently gave the girdle and locket to the sister of Helain de Taningues.
Vulgate VII names a 'Helyes li chatelains de Roestoc'. The "li" is masculine, and Helyes appears to be one of the rebel kings, possibly killed in battle, which makes the Lady of Roestoc either his widow or his orphaned daughter.