Eugène Viollet-le-Duc was a French architect of the 19th century known for his focus on the restoration of historical buildings. He is considered an influential figure in the field of architecture and was a pioneer in the field of restoration theory. Viollet-le-Duc believed in the importance of preserving the original elements of buildings during restorations, and he also introduced the concept of "congregational restoration," which involves completing missing parts with new elements that clearly differentiate the old from the new.