Original: It was founded in 910 AD when the Christian princes of Asturias along the northern coast of the peninsula shifted their main seat from Oviedo to the city of LeÃ n/O. The County of Portugal separated to become the independent Kingdom of Portugal in 1139 and the eastern , inland part of LeÃ n was joined to the Kingdom of Castile in 1230 .
Simplified: It was founded in 910 A.D. . The Atlantic provinces became the Kingdom of Portugal in 1139 , and the eastern , inland part of the kingdom was joined to the Kingdom of Castile by 1230 .