The monastery of San Pedro de Cardeña is a Trappist abbey located in the municipality of Castrillo del Val, 10 km from the centre of Burgos.

The monastery must have been founded before 902 when the Count of Lantarón and Cerezo, Gonzalo Téllez and his wife Flámula made the first documented donation to the monastery on 24 September of that year of a serna in Pedernales and some salt threshing floors.

In the 9th or 10th centuries, its monks were martyred by the Muslims, canonised in 1603 and known as the "Martyrs of Cardeña". The monastery enjoyed great popularity with a large number of devotees, including King Philip III of Spain and his wife Queen Margaret of Austria.

One of its precious relics, the head of its abbot Saint Stephen, was transferred to the Monastery of Celanova; there are also two urns in the Monastery of La Huelgas and another in the Cathedral of Burgos.

The prosperity of the monastery in the early medieval period is reflected in the quality of its scriptorium, in which the monk Endura produced extraordinary works.

From the chapterhouse, which dates from the 13th century, the Romanesque cloister, which dates from the 12th century, can be seen through large windows. It consists of semicircular arches on single columns resting on sturdy shafts and crowned with capitals imitating the Corinthian style. The decoration of the arches is reminiscent of those of the mosque of Córdoba due to their polychromy, alternating the colours white and red. On the left wall there are ancient stones whose inscription commemorates the tragic event.

On the right side of the Gothic church, there is a Baroque chapel dating from 1753 to which the remains of the Cid Campeador and his wife Jimena were transferred. On the walls of this room, known as the "Chapel of the Heroes", there are 29 niches with inscriptions with the names of kings and relatives of the Cid.