The church from which this apse comes was probably part of a castle complex, built by Christians engaged in the reconquista, the campaign to wrest the Iberian Peninsula from the Muslims. The great round arch, thick walls, barrel vault, half-dome, and small windows are typical of the Romanesque style. Built of golden limestone, the apse is decorated with sculpture on the interior and exterior. Inside, Saint Martin tramples evil beasts on the left, and on the right the angel Gabriel announces the impending birth of Christ to the Virgin Mary. The capitals surrounding the slitlike windows are carved with contorted animals and birds.

A fresco showing the Virgin as Mother of God, flanked by archangels and attended by the three magi, is installed in the half-dome. Dating to 1140, it comes from the Church of San Juan de Tredós, in Catalonia.

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