The church originates from the 10th century and was built using ashlars and slate. It has a double aisled nave separated into four sections. The central section is the widest and the triple East end has semicircular apses. There are three Romanesque entrances: one on the western gable end and two others on the nave of the Epistle Side.
The nave is barrel vaulted while the aisles are covered by groin vaulting or simple rib vaulting. The Northern side of the transept is decorated with a blind arcade.
The western doorway is from the Renaissance period and is finished off by a bell cote from the 18th century. The eastern doorway that gives access to the church and the cloister (which no longer exists) is Romanesque. The door on the southern side is now walled up. It possesses a double round arch over plain jambs. On this side, the ancient cloister stood. It was substituted for one from the Late Gothic period in which today, only remains exists.
The majority of the sculptures are concentrated on the capitals in the windows and pillars in the nave. They are decorated with geometric and plant motifs. There are two auxiliary altars in the apsidiole sections, one on the Epistle Side and the other on the Gospel side. Both have niches for the relics.
After the previous studies were carried out, the technicians determined that the first phase of restoration in the church will focus on the cleaning, rehabilitation, and improvement of the roof. Then they will proceed to eliminate dampness and moisture. Problems with moisture provoked the deterioration in some of the interior parameters of the building.
Thus, the roof will be substituted in the apses, apsidioles, and sacristy, where it will be waterproofed.
Once the intervention on the roof is finalized, they will proceed to clean the walls and roof in the sacristy and the north wall, eliminating existing plants and renovating the floor of the sacristy.
To complement the intervention, a series of sensors have been installed in the interior to monitor the structural conditions of the church. The sensors will register necessary data to establish a diagnostic analysis which will lead to the execution of the second phase.
The action has focused in adapting the interior of the sacristy so that the cult can happen during the cold seasons, given that it is impossible to provide the space with the necessary environmental conditions. Therefore, the intervention consists in the introduction of a module of new design made up of all the necessary elements to conduct all the liturgical celebrations, introducing seating heated through a bright sheet under the shale slate and lighting the space adequately. On the other hand, the sacristy was affected by a humidity problem that had been necessary and urgent to fix, through actions like the rejoining of the facing, the isolation of the counters or the installation of new windows that allow for the air to circulate automatically, since they are synchronized with the environmental sensors of the MHS system.
On the other hand, this second phase has also given new lighting to the areas of the apses and the transept, a more efficient one and in accordance with a building of the monumental characteristics of San Martín de Castañeda.
The intervention on movable property had two different actions. One of these has focused on the restoration of a simple set of mural paintings on the lateral absidiole, dated from the first half of the 17th century. The paintings, which presented a very bad conservation state, simulate a dome with gallons, in a very restrict black and red color palette. As for the second action, in order to reinforce the sacred character of the new space of the sacristy as a liturgical space, and thanks to the arrangements of the priest, the parish has obtained in deposit from the Museo de los Caminos de Astorga, a romanesque carving of the Madonna with the Child, dated from the 13th century. An interesting piece that was restored in the atelier of the Fundación Santa María la Real before its permanent placement in this new space.
