The room that is now the mausoleum was once the castle’s private chapel, remodeled in the 16th and 17th centuries. From the 15th century up to the events of the War of Spanish Succession, the remains of Papa Luna were laid to rest here. In the 16th century, the Renaissance portal leading into the room was built: a round arch resting on fluted columns and decorated with Renaissance-era motifs including festoons, griffins and cornucopia.

The mausoleum itself was constructed in the 17th century, featuring the heraldic representation of the coat of arms of Benedict XIII in polychrome plasterwork in the pediment. Upon entering the castle-palace, there is a monumental staircase erected in the 17th century in the space that had held the open medieval courtyard up to that time, which leads to the rooms surrounding the courtyard. This imposing staircase was built in 1665 and features a passageway of round arches at the top, resting on simple columns with a base and capital, covered by a large dome on pendentives topped by a lantern. There is also Baroque plaster decoration.

In the Sala de la Corona de Aragón, a space constructed in the 17th century, the most striking feature is the wood frieze decorated with leaf, egg and die motifs and turned colonettes. Covering this room is a simple flat ceiling with clad wood beams and plaster fill. On the exterior of this immense prism-shaped volume with four cube-shaped turrets at the corners is the portal, located on the main façade: broken down into three levels, like the rest of the building, it is framed by two circular turrets. Both are made of masonry up to the level of the arched passageway and contain three blind arches each, topped by six openings around the perimeter of the upper section.

Projecting some two meters beyond these turrets, the entrance consists of a round arch flanked by double pilasters with Tuscan capitals holding up a decorative frieze of mascarons and a broken pediment of volutes, also framed by Tuscan order pilasters. Over the entrance there is a second volume that is more severe and austere than the first but uses the same compositional

scheme, and above that is the third volume containing an undecorated opening featuring an architrave with an overhang.

In the 16th century, Pedro Martínez de Luna, the first count of Morata and viceroy of Aragon, ordered the transformation of the fortress, giving it a more palatial appearance and adding to this magnificent structure an arched passageway running around the entire upper perimeter and a no less elegant main portal which, according to experts, was inspired by the main façade of the Ducal Palace of Urbino (Italy), highlighting the influence of the Italian Renaissance in the territory of Aragon.