He was assisted in several of his projects by the architect Eugène Viollet-le-Duc. Viollet-le-Duc was twenty-six, and had studied mathematics and chemistry but not architecture; he learned his profession from practical experience and travel. In 1840 he worked the first time for Mérimeé; in one month he designed a solution which prevented the collapse of the medieval Vézelay Abbey. In 1842–43, Mérimée gave him a much more ambitious project, restoring the facades of the Cathedral of Notre-Dame de Paris. He returned the statues which had been removed during the French Revolution, and later restored the spire.