 Complete the following phrase:  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 lEstree, 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 