On the 21st of December 1063, King Fernando (Ferdinand) I and Queen Sancha consecrated the old church, of which the northern and western walls and the western door (bricked up) have been preserved to the present day. Their daughter, Urraca, commissioned the construction of most of the current church that, after her death in 1101, was continued by Alfonso VII and his sister, la infanta Sancha. The works were directed by the architect Pedro Daustambem 

The temple is formed of three naves and three apses. The Capilla Mayor (Main Chapel: Chancel) is a space covered by ribbed vaulting with lancet arches and is the work of Juan Badajoz ‘El Viejo’ (‘the Old one’). The altarpiece is from the 16th century and depicts a wide range of iconography: scenes of the life of the Virgin, the Passion of Christ, and the life of St. Thomas and his preaching. As a privilege since time immemorial, the Blessed Sacrament is permanently displayed in the center of the altar. Inside the silver-plated shrine, there is a monstrance and underneath, the reliquary casket with the remains of St. Isidore from 1847. Columns, doors and windows are adorned, both internally and externally, with capitals, chess-like fascia, plant and braided motifs and so on.

The main door is called Puerta del Cordero (‘Door of the Lamb’) since this is the leading topic represented on the tympanum: the Mystic Lamb supported by angels, the sacrifice of Isaac, Sarah outside her tent and Ishmael the archer riding through the desert. On the left, the figure of San Isidoro (St. Isidore) and on the right, San Pelayo (St. Pelagius). Finishing off this iconographic work are King David, five musicians and the signs of the Zodiac. La Puerta del Perdon (‘Door of Pardon’) is the name that was given to the entry to the southern transept and is devoted to the pilgrims. A dog and a lion, watching over the temple, support the tympanum, which shows the Ascension, the Descent from the Cross, the Three Marys at the Tomb and the Ascension. St. Peter and St. Paul also appear, each on one side of the arch.

However, what has always drawn Leonese people towards their Basilica is the constant presence of the Blessed Sacrament, solemnly displayed and granted as a privilege since time immemorial. The date of its arrival remains uncertain but there is evidence of its being already there by the 16th century. This implies that the Basilica is kept open nearly all day and night to worshippers.