Gwynedd is one of several Welsh successor states that emerged in 5th-century post-Roman Britain. It was based on the former Brythonic tribal lands of the Ordovices, Gangani, and the Deceangli which were collectively known as Venedotia in late Romano-British documents. Between the 5th and 13th centuries Gwynedd grew to include Ynys Môn and all of north Wales between the River Dyfi in the south and River Dee (Welsh Dyfrdwy) in the northeast. The Irish sea (Môr Iwerddon) washes the coast of Gwynedd to the west and north and lands formerly part of the Kingdom of Powys border Gwynedd in the south-east. Gwynedd's strength lay in part due to the region's mountainous geography which made it difficult for foreign invaders to campaign in the country and impose their will effectively. Popular tradition attributed to Nennius, a 10th-century Welsh chronicler, traced Gwynedd's foundation to Cunedda. According to Nennius, Cunedda migrated with his sons and followers from Brythonic Lothian, in southern Scotland, in the 5th century.