The Basilica of Santa María de Arcos, an architectural treasure of early Christian origin, stands on the outskirts of the town of Tricio. This temple, which dates back to the 5th century, is considered the oldest monument in La Rioja, and has fulfilled its religious function uninterruptedly from its construction to the present day, which underscores its historical and spiritual importance in the region.

Some historians, such as Caballero, Arce and Utrero, suggest an alternative theory about its origin, proposing that the temple could have been built during the Reconquest, in the 9th or 10th centuries, based on technical and historical analysis.

The temple stands on the remains of a Roman mausoleum from the third century, and in its construction various pieces of this and other buildings of the ancient Roman city known as Tritium Megalon were reused, which gives it a unique character and a deep link with the Roman history of the area.

The Basilica of Santa María de Arcos has a basilica structure with a longitudinal plan of three naves and a quadrangular chevet. The building, constructed in the 5th century, takes advantage of the base of the old Roman mausoleum of the 3rd century, which occupied the area of the presbytery or Holy Chamber. Inside, the side naves are separated from the central nave by arcades resting on Corinthian columns. These columns, which stand out for being the largest in diameter in Spain, reach an impressive height of 20 meters and were made from fragments of a Roman building of the first century.

The arches that support the structure are mainly Visigothic, dating from the late 6th and early 7th centuries, and are built of tuff stone. However, the first arch of the arcade on the gospel side is of Roman type, made of sandstone, characteristic of the region.

Originally, the temple was located on top of the mound that Tricio occupies today. During the 18th century, the interior of the basilica was decorated with baroque plasterwork, on which the coat of arms of the Order of the Terrace can still be seen.

The Virgin of Arcos, patron saint of Tricio, takes her name from the arches of the basilica. Her feast day is celebrated with great devotion on the first Sunday of May. Her image, enthroned and with the Child in her arms, is in a niche at the head of the church. The current image is a life-size reproduction, since the original carving, a pre-Romanesque brown Virgin dating from the 11th century, is kept in the parish church of Tricio.

Archaeological excavations carried out between 1980 and 1983 revealed the existence of numerous burials under the floor of the basilica. The oldest date from the 5th and 6th centuries, and although they are Paleo-Christian tombs, several of the sarcophagi are reused Roman, belonging to the 1st to 3rd centuries AD.

Medieval tombs were also found, of simple structure, made with stone slabs without a lid. In addition, Roman funerary stelae from the 2nd century with interesting inscriptions were found, brought to the site in the 5th century to be used in the foundation of the basilica. Among the most remarkable finds is an early Christian stele in Carrara marble, decorated with a Chrismon, a Christian symbol. This stele, imported from Italy, is the only one of its kind in La Rioja and the oldest in the region.

At the head of the church there are remains of the original paintings that decorated the basilica. These paintings, in Romanesque style and dating from the end of the 12th century, have a simple and rough style, and were applied over the original Paleochristian decorations from the 5th century. The scenes depicted include episodes from the Passion of Christ, although some are fragmented.

On the lower part of the walls there is a plinth decorated with geometric drawings in the Mozarabic style, also from the 12th century. In addition, in the corners of the east wall there are remains of an early Christian mosaic that, in its time, covered the Holy Chamber, providing further evidence of the artistic wealth that adorned this ancient temple.