In the town of Castillo we highlight the Church of St Peter. It dates from the 12th century, since in 1210 it is mentioned in the documentation of the cartulary of the Monastery of Santa María del Puerto de Santoña.
It has a Latin-cross plan with a single nave of two bays, to which small chapels open in the wall. The crossing bay is flanked by two chapels; preceded by a slightly lowered round-headed triumphal arch, it shows a semicircular plan and is covered by a barrel vault with decoration of fluted grooves in the form of a scallop (venera). The rest of the vaults have tierceron and curved ribs (combados). The structure displays all external walls in ashlar masonry.
On the south wall of the last bay of the nave, the entrance doorway opens, which is the most characteristic element of the building. It is splayed with a slightly lowered round arch, and is bordered by roll mouldings (baquetones) that rest on the small polygonal bases characteristic of the late Flamboyant Gothic of the late 15th century. The drip moulding (guardapolvos) has floral decoration on its face, as well as rolled vegetal tendrils on its soffit/back (trasdós). The apex of the conopial arch is crowned with a finial in the form of a fleur-de-lis. The doorway is laterally framed by two decorative buttresses with finials. This doorway follows a Hispano-Flemish model, which has some importance in the Trasmiera region.
Both the transept and the apse are the result of a Renaissance refurbishment carried out between the late 16th century and the early 17th century.
In 2013 it was declared a Site of Local Interest.
