Dedicated to Saint Thyrsus, it was built by Tioda, the royal architect of Alfonso II of Asturias.

The Great Fire of Oviedo in 1521 and rebuilding in the 18th century removed most of the original church, except for a three-light window.

It is that of a basilica with nave and aisles divided by rude stone piers set at unequal intervals, from which round arches spring.

In the easternmost bay, however, owing to the smaller span, the arch was made sufficiently pointed to raise its crown to the same height as the others.

With its pre-romanesque bases, rough brick arches, and capitals with rude packed leaves, it gives an idea of the better style of building and carving in the time of Alfonso II of Asturias.