In the beginning of 1864, he celebrated the conclusion of his most important project, the restoration of Notre-Dame. In January of the same year he completed the first phase of the restoration of the Cathedral of Saint Sernin in Toulouse, one of the landmarks of French Romanesque architecture. Napoleon III invited Viollet-le-Duc to study possible restorations overseas, including in Algeria, Corsica, and in Mexico, where Napoleon had installed a new Emperor, Maximilien, under French sponsorship. He also saw the consecration of the third church that he had designed, the neo-Gothic Church of Saint-Denis de l'Estree, in the Paris suburb of Saint-Denis. Between 1866 and 1870, his major project was the ongoing transformation of Pierrefonds from a ruin into a royal residence. His plans for the metal framework he had designed for Pierrefonds were displayed at the Paris Universal Exposition of 1867. He also completed the tenth and final volume of his monumental dictionary of medieval architecture. He also began a new area of study, researching the geology and geography of the region around Mont Blanc in the Alps. While on his mapping excursion in the Alps in July 1870, he learned that war had been declared between Prussia and France.