It's a late-Roman sanctuary of the third century built 14 kilometres from Antigua Lucus Augusti. It was dedicated to the goddess Cibeles and reconverted to the cult of Santa Eulalia. On the outside. a small atrium with two columns *in antis* precedes the façade. in which a door with a horseshoe arch is opened. a typology that would later be adopted by the Visigoths. It is the oldest horseshoe arch existing in Spanish architecture as a structural element. as it had previously only appeared in the decoration of some Roman stelae. It is considered a *unicum*. that is to say. there is no other building of the same characteristics in all the territory occupied by the Roman Empire. It is one of the most singular constructions of all Spanish architecture. The original building consists of a kind of double vaulted crypt. with a structure very similar to the Crypt of Saint Antolin in the Cathedral of Palencia. square in shape. about 6 m. per side. alabaster sediment with a pool in the centre. which has at one end a quadrangular niche of about 3 m wide by 1 m deep and on the opposite side the entrance with a rectangular narthex and a portico in front with two columns that supported three arches now disappeared. the central one wider. The building was later modified inside. reorganising it in a longitudinal sense by adding two rows of three arches. so that each row rested on the walls and on two columns added for this purpose. It is worth mentioning the rectangular windows on the main façade. made up of stone blocks in horizontal rows. with a discharging arch. which flank the door. The latter has a brick horseshoe arch in a radial arrangement. prolonged by a quarter radius. which would have been much less surprising in a Visigoth church of the seventh century. On some of the stones of the façade can be seen a sculpted decoration. clearly from the same period of the original construction. consisting of dancing figures that have nothing to do with Roman sculpture. The barrel vault. covered with stucco. retains a decoration based on geometric. vegetable and animal motifs that. while preserving the Roman pictorial tradition. also reminds us. above all in the drawings of birds. some of the decorations sculpted in later Visigoth churches such as Quintanilla de las Viñas and whose great influence can be seen in Asturian painting in the ninth century. It can be concluded that the building was originally a nymphaeum dedicated to some pagan divinity between the fourth and fifth centuries. since the only thing that opposes this theory is the existence of the horseshoe arch. but we know that these had previously appeared as decoration in some Roman stelae in northern Spain. According to this. the sculpted decoration could have Celtic influences due to the style and clothing of the dancers. This theory is based on the existence of the pool and a complicated water conduction system that. entering through the bottom niche. maintained the water level in the pool. which suggests some kind of religious rite related to water. The ensemble is a singular example of Hispano-Roman syncretism. In the later modification of the building. the only nave was divided by adding two rows of three arches. which rested on four columns with capitals. two of them attached to the walls of the portico and the chancel. Each of these groups supported a separation wall. which in addition to reinforcing the vault. gave it a basilical structure of three naves. possibly with the intention of having a greater appearance of church. for which the pool was also filled.