The San Martin church was founded by Mrs. Mayor of Castilla, widow of Sancho III of Navarra, and its construction is thought to be around 1066. It is thought to have taken between 15 and 20 years to construct. On the outside, the head of the church stands out, composed of three apses: a central, semi-circular larger apse and two smaller ones. The exterior surface of these apses have a series of columns and some checkered projections which serve to enrich the aesthetics of this section. On each of the sides the facade is simple, with the exception of the North facade, which is flanked by columns with capitals, even though they have deteriorated being North facing. Towers at the foot of a church are not common in Romanesque churches, and might reflect a German-Romanic influence. The dome stands out, and is unusual in that it is octagonal whereas most other churches with domes in Palencia are quadrangular.
The decorative elements are concentrated in three areas
- The checkered, which runs along the whole surface of the church at different heights
- The capitals, situated at the sides of the windows and at the end of the columns, decorated with plant, animal and human personalities. As with the interior capitals and the beam heads, there is an important symbolic element in them aimed at transmitting a message related to Christian doctrine.
- The beam heads are the little sculptures that decorate the eaves of the roof. In all there are 309 figures in vegetable, geometric, animal monstrous and human representations.
Inside the church the first thing to be noticed is the sobriety of the building, expressed in the nakedness and simplicity of the walls. The floor plan corresponds to the basilica type. The body of the church has three longitudinal naves ending at the head of the three apses whose volume is equal to the naves they enclose, two small, lateral apses and a big central apse. This is the only place where there is free-standing sculptural decoration: on the left, an image of Saint Martin from the 14th century; to the right an image of Santiago from the 16th century, and in the centre a Crucified Christ image from the end of the 13th century. The great size of the windows in the apses also stands out.
The naves of the church are covered by vaulting divided into various sections by moulding arcs whose function is to receive the weight of the ceiling and transport it to the pillars and to the walls, which are very thick, as can be seen through the enbrasures of the windows. Although of basilica construction, through the structure of the ceiling of the naves can be seen movement towards the Latin cross plan. The nave and the central apse form the longitudinal axis of that cross and this axis is cut perpendicularly by the transversal nave.