Several Early Welsh saga poems, known collectively but rather misleadingly as Canu Llywarch Hen ("The Poetry of Llywarch Hen"), survive that describe this Brythonic/early Welsh kingdom, however they were probably not composed until some time after the events alluded to and are to some degree mythologised.
Cynddylan apparently joined forces with king Penda of Mercia to protect his realm, and together they fought against the increasingly powerful Anglian Kingdom of Northumbria at the Battle of Maes Cogwy (Oswestry) in 642.