The parish church of St Michael the Archangel, in the highest part of Castiello de Jaca, towers over a cliff which dominates the Aragón river valley. Its origins are Romanesque (12th century) and it was composed of a single nave facing east with a semicircular apse. It still maintains the sturdy and imposing character of Romasque churches despite having undergone considerable modification. Between the 13th and the 18th centuries a choir balcony and an atrium were built at the western end. New lateral chapels were also added.

Viewed from the outside, the bell tower on the east side stands out from the rest of the building. The apse is protected by a quarter spere vault, the small chancel has a barrel vault and the main body of the nave has a ribbed vaulted ceiling divided into three parts and reminiscent of ceiling in the main nave of Jaca cathedral. Likewise, the choir and the portico also have cross vaults. The chapels closest to the chancel have barrel vaults, the second chapel on the Gospel (left) side a barrel vault with lunettes and the second chapel on the Epistolary (right) side has a cross vault.