This Gothic church-fortress has a broad and spacious nave and a polygonal sanctuary. It is covered with cross vaults which are supported by means of sturdy exterior buttresses, and there is a tower-belltower beside the sanctuary.
It has two doorways, of which the western one is the most interesting. Its large pointed archivolts are supported on columns and historiated capitals, and the tympanum features a representation of the kings of Navarre and Saint Saturnino.
In around 1300, the interior of the sanctuary was decorated with wall paintings attributed to the artist known as the Master of Artajona, which are archaic in style and inspired by Romanesque-Byzantine art. These murals depict the Day of Judgement and the apostles, and today are conserved in the Museum of Navarre. In 1340, Roque de Artajona added a frieze of Gothic paintings dedicated to San Saturnino in the presbytery.