The “Plan de Rome” of Caen (France) is a large plaster model of nearly 70 square metres which represents ancient Rome in the 4th century A.D. Classified in the Inventory of Historic Monuments, it is the work of the architect P. Bigot (1870-1942), winner of the Grand Prix of Rome and professor at the Ecole des Beaux-Arts in Paris. The Plan de Rome is an exceptional piece of our cultural heritage whose only equivalents are its twin which is in Brussels (Belgium) and the model made by I. Gismondi which is in Rome (Italy). Bequeathed to the University of Caen, this relief plan is the pretext of a pluridisciplinary project for a virtual 3D model. Work on developing a virtual model of the city of Rome in the 4th century A.D. began in Caen in 1994. The work involves the development of a fully interactive model of ancient Rome that includes both its architecture and the machinery in use. In reconstructing the past, scholars will be able to test their assumptions from the architectural and topographic viewpoints and test machine functionality. The visitor to the virtual model will be able to wander through a full-scale city and enter most of its large public monuments and a few residences. The work preliminary to the reconstruction itself involves the analysis of ancient source materials, which is currently being performed by the project team’s members who have recourse to outside experts for help with specific points. The body of source material is directly accessible via the interactive model. This paper discusses the history of the project, its methodology and the problematic of the reconstruction of an entire city in interactive virtual reality.