Old St. Peter's, founded by Emperor Constantine, was the largest cult building and the most important pilgrimage site of early Christian Rome. The dating of the construction, its architectural form, and its furnishings have recently been the subject of repeated discussion. Hugo Brandenburg, one of the foremost experts on early Christian architecture, summarizes his years of research and attempts to provide an answer, illustrated through drawings and simulations, to how the basilica presented itself to visitors and pilgrims in the 4th century AD.