What is "bardulia"

According to some sources, Bardulia is the ancient name of the territories that composed the primitive Castile in the north of what later became the province of Burgos. The name comes from Varduli, the name of a tribe who, in pre-Roman and Roman times, populated the eastern part of the Cantabrian coast of the Iberian peninsula, primarily in present-day Guipúzcoa. Some assert that the Varduil also encompassed or assimilated the Caristii and Autrigones.

It has been speculated that a possible expansion of the Basque territories— Late Basquisation, an expansion to the Basque Country in the 6th through 8th centuries—occasioned a westward migration of the Varduli to what the documents of the Low Middle Ages call Bardulia.