time questioned the cathedral's ability to remain standing. This almost implausible structure, together with the poor quality of stone and foundation, caused it to suffer constant interventions and restorations since the 10th century, making the church a European paradigm of transformation intervention, restoration, and conservation. Modifications and restorations Extensive modifications and additions have been made to the cathedral, sometimes trying to improve its stability but also to adapt it to contemporary taste, until restorations beginning in the nineteenth century tried to remove most non-Gothic elements and restore the building to its original, purely Gothic state. In the fifteenth century, the south tower was finished, in the Flamboyant Gothic style. Also in this style was the library, currently the Chapel of Santiago, built by Juan de Badajoz el Viejo at the end of the century. In the sixteenth century, Juan de Badajoz started the addition of non-Gothic elements with the construction of a plateresque gable in the Western façade, too tall and heavy. He also added a plateresque sacristy in the south-east end of the cathedral and rebuilt some vaults. Serious problems started in the seventeenth century. Parts of the largest vault in the crossing fell down in 1631. It was replaced by a baroque semicircular dome designed by Juan de Naveda. This addition damaged the delicate balance of the whole building, causing cracks in the southern façade that needed restoration in the end of the century, including a new gable. In the eighteenth century, Joaquin de Churriguera tried to improve stability by adding four pinnacles around the dome, but this caused further damage. The cathedral was affected by the Lisbon earthquake of 1755. Extensive rebuilding of the southern façade was done. In 1857, stones started to fell down from the central nave and the crossing, causing fears of a complete collapse. Matías Laviña started a full restoration of the building in 1859. He dismantled the central dome and the pinnacles, parts of the transept and the southern façade. After Laviña died in 1868, Juan de Madrazo was commissioned to direct the restoration. He was a friend of Viollet-le-Duc and had a good knowledge of the French Gothic style. The goal of his extensive work was making the Cathedral stable and restoring its original pure Gothic style. He placed a complex wooden support structure to keep the vaults in place and proceeded to rebuild the vaults and the southern façade. When