When I was there it fell mainly in the brief strip of land between the mountains and the sea.
But in the Asturias, although less splendid than the greatCathedral at Burgos, there are a number of ancient churches (also collectively a world heritage site) built for the celebration of the Visigothic (Mozarabic) liturgy. Here are illustrations of one of them - the church of Santa Cristina de Lena south of Oviedo (if my notes from that holiday are accurate!)
The Mozarabic rite called for an iconostasis to be
placed before the main altar, in the central apse. The gifts of bread and wine would be prepared in one of the side apses, and then Offered at the other before being brought to the main altar for the consecration. The iconostasis, richly carved and only partially remaining, would have obscured the actual celebration of the mysteries from the view of the laity. At the west end of the church is a gallery for local notables - a feature common in the Westwerken of imperial German churches and also found at a number of places in England.
The exteriors of the churches are generally unadorned, as can be seen in this photograph. When I was there, although the sign said the church was open, in fact it was locked, and it was only after I and a couple of other visitors had been hanging around for a while that a lady appeared bearing the keys to allow us in to the building.
Also of note in the area are the churches just outside Oviedo itself - one of which was originally a summer residence for King Ramirez of Asturias.
Not directly concerned with this, but worthy of note as the oldest church in Spain is the little church of St John the Baptist just outside the city of Palencia. St John the Baptist uses Roman materials in its construction and although no longer in use as a church it is a testimony to the antiquity of Spain's Christian heritage.