The feature wall is strategically positioned to illurninate the altar for one hour on a particular day each year. The fortress at Castielfabib is located Qn a hilltop that dominates the town and that has been formed in a meander of the river Ebró. The church itself occupies the fourth floor of one of the castle‘s towers, overlooking a steep decline on the western slope of a rocky outcrop.

Access to the fortress is by way of a passage at the height of the third floor of the tower in which buried bodies have been unearthed, such as that of a woman and her baby discovered with their clothes almost intact. The passage leads from the town square to the opposite side of the outcrop, where the access is isolated from the rest of the population. Gothic paintings and carved capitals depicting religious figures from the period have been found within the building, including the head of a warrior monk from the Order of the Temple, to which the church belonged.
 
The fortress dates back to the early 14th century, which saw the construction of the single-nave altar, the former weapons room, which was rectangular with flat chavet walls. The interior was divided into four spaces separated by three pointed arches, characteristic of the early Gothic style. In between the buttresses, side chapels were built, with groin vaults.
 
Following the earthquake of 1656, huge reforms were carried out, which saw the construction of the communion chapel with its cupola and the atrium at the entrance, which were designed with completely different ceilings. The flat header wall was removed in its entirety and a presbytery was built in its place.
 
At the foot of fhe church, on the Epistle side, twin chapels were erected — the Dominican chapels — with a simulated star-shaped groin vault adorned with sculpted cherubim heads. The old choir stall was removed and its oculus sealed. The first bell tower disappeared, and a new one was built adjoining it, but independent and separate from the church. Curiously, the chevet on the feature wall is strategically positioned so that it illuminates the altar for one hour on a particular day each year.
 
The latest reforms were made during Spain‘s Carlist Wars in the 19th century, apparently to rapair the damage it suffered during the conflict. It was transformed into a three-nave church and the supports for the lancet arches were removed to the new vaults of the lateral naves. The presbytery was extended once more, eliminating the passage between the two sides of the fortress, and it was covered with a cannon vault, upon which stands an octagonal lantern. It was at this time that the bell tower was faced in brick.