Large basilical church, Romanesque architecture, rectangular in plan, with pointed barrel vault (cradle) raised over a small toral arch, with the main nave closed by a simple apse.

It is a simple space based on the Cistercian austerity postulates: stone masonry, absence of ornaments, with only the slight vaulted note standing out.

The temple is illuminated by three small windows, one of them located in the center of the apse, it also has a small sacristy.

It was a chapel for the Knights Templar and then for the Montesians, it was the papal basilica of Benedict XIII and his successor, Clement VIII (also considered Antipope and who occupied the papal dependencies until his abdication in 1429).

It was the burial place of Pedro de Luna (Benedict XIII) between 1423 and 1430 (he was buried next to the apse, on the right, under a stone slab (former altar of the Templar altar), without any inscriptions). He was then moved to the Luna castle-palace in Illueca (in the province of Zaragoza), where he was born – a tribute paid to his memory by some family members.

The church was dedicated to the Virgin Mary (“Humil Verge Maria”) and the Three Kings of the East. The church was built in the 13th century by the master Templar builder, Friar Berenguer de Cardona. On the outside, above the entrance to the temple, the five thistles of the Coat of Arms of the Cardona family were engraved, in memory and honor of this ancient and very noble lineage.

In 1923, on the fifth centenary of the death of Dom Pedro de Luna, the University of Zaragoza wanted to pay tribute to him, placing a black marble tombstone in front of the door that gives access to the Plaza de Armas, the Papal Shield, the Aragonese Coat of Arms, with the following caption: "Aragon asks you to pray to God for Benedict XIII, Pedro da Lua, the great Aragonese with a clean, austere, generous life, sacrificed for an idea of duty. The final judgment will discover the mysteries of History . In him Jesus Christ and Saint Mary, his mother, save us". (free translation).