Three worship spaces occupy the same Roman footprint on Via Labicana, stacked across twelve centuries of continuous sacred use. The current basilica dates to 1108, built directly above a fourth-century basilica, which itself rests on a first-century Roman house and Mithraic temple. Excavations begun in 1857 by Father Joseph Mullooly revealed the complete stratigraphic sequence intact — an archaeological rarity in a city where medieval builders typically cannibalized earlier structures for materials. The twelfth-century upper church preserves one of Rome's finest medieval mosaic programs. The apse composition depicts the Triumph of the Cross as a Tree of Life, its acanthus scrolls populated by saints, doctors of the church, and everyday Romans drawing water. Cosmatesque floor panels from the Cosmati workshop showcase geometric precision in salvaged marble. The chapel of Saint Catherine houses Masolino da Panicale's frescoes of her martyrdom, completed in 1431 and among the earliest Renaissance works in Rome. Descending to the fourth-century basilica, visitors walk the nave where early Christians gathered during the final persecutions. Fragmentary frescoes depict the Legend of Sisinnius, an eleventh-century narrative painted in vernacular Latin rather than classical forms — one of the oldest examples of written Italian. The basilica honors Clement I, fourth pope and author of the earliest Christian text outside the New Testament canon. His supposed relics rest in the upper church, brought from Crimea by Saints Cyril and Methodius in 867. The lowest accessible level reveals a first-century insula and a Mithraeum carved from its ground floor around 200 CE. The triclinium retains its stone benches where initiates reclined during ritual banquets. A relief of Mithras slaying the cosmic bull occupies the altar niche, flanked by torch-bearers Cautes and Cautopates. The cult's all-male membership and secretive rites made it a rival to early Christianity among Roman soldiers and merchants. Water still flows through the Mithraic chamber, channeled from the same spring that supplied the ancient neighborhood — a functional reminder that this site has never ceased being a threshold between surface and depths.