During the two centuries of the work on the monastic church, there was a change between the Romanesque and Gothic architectural systems, a circumstance that gave pause to the master builders, with some rectifications to existing parts.
The first construction phase, in the 12th century, included the apses and the first two sections of vaults, separated by rounded arches. The works then stopped for about half a century and were resumed with Gothic forms, distinguished by pointed arches and much larger windows. A lantern-shaped octagonal crossing tower rises above the Romanesque transept, covered by the intersection of four pointed vaults closed by a keystone, which contains the image of God in Majesty. The windows are of a very early Gothic style with tree-shaped divisions.
The best example of the transition from Romanesque to Gothic is in the differences in the location of the impost of the formerets on each of the four pillars of the transept. Looking at those closest to the apse, the impost of the right formeret is lower than the one on the left. On the opposite side, the right formeret maintains the lowest level, indicating that it follows Romanesque style. However, the pillar extends in height and a second Gothic impost appears at the base of the pointed arches.
All the changes made during the transition from one construction system to another involve changes in the structural behaviour of the last three sections; in addition, to compensate for the greater pressure, a fourth single nave was added inside and aligned with the bell tower outside.