Inland, the area is characterized by the smoothness of the landscape, mainly formed by fertile valleys with a small river network that crosses the county.

Castrillón appears named as Castilione in the Liber Testamentorum of the Cathedral of Oviedo, sourced as a copy of a document from 857 AD.

In these documents, and others from the 10th century, there are references to the various contributions of the Kings to the Church of Oviedo, like the one dated on 921 where the donations from Ordoño II, including the Sancti Saluatoris de Petris Albis church (San Salvador de Piedras Blancas).

According to tradition and written legend, the Cruz de la Victoria (Victory Cross) was forged there.

Under the protection of the castle, there was a monastery that gave shelter to the orders of Santiago, San Francisco and La Merced.

The salt industry was very important during the Early Middle Ages, in the areas of Bayas, Naveces and Santa María del Mar.

The construction of houses for their employees was the beginning of the current population centres of San Xuan, Arnáu, and Salinas.

At one point, there were so many Asturian immigrants from the Castrillón area to WV that there was a Spanish consulate in Clarksburg[3].