Standing next to the Peristyle is the Cathedral of Saint Domnius, which represents one of history’s greatest ironies. This building was originally designed as Diocletian’s grand octagonal mausoleum—a resting place for an emperor notorious for his brutal persecution of Christians. Yet, by the seventh century, that very tomb was transformed into a Christian cathedral. Today, it is recognized as the oldest Catholic cathedral in the world still using its original structure. If you head below ground into the basements, or cellars, you might feel a sense of recognition. These vaulted stone halls served as the filming location for the dragon pits in the television series Game of Thrones. These substructures are perfectly preserved because, for hundreds of years, they were used as a literal trash dump by the residents living upstairs. Ironically, this debris shielded the Roman stonework from weather and from being dismantled for building materials, leaving us with one of the best-preserved Roman architectural complexes in the world. Accessibility is generally good near the main entrances, though the basement floors can be uneven.