Beneath the medieval streets of Grenoble's old town lie the foundations of Cularo, the Gallic settlement that became a Roman town. The most visible remnant is a section of the late 3rd-century defensive wall, built under emperors Diocletian and Maximian — a massive construction of limestone blocks and tile courses that still stands several metres high.
The walls can be seen partly in situ, incorporated into the fabric of later buildings, and partly in archaeological sections near the cathedral. The site adjoins the Musée de l'Ancien Évêché, where the 4th-century baptistery below the bishop's palace forms part of the same Roman-era urban layer. If you walk the old town carefully, you can trace the outline of the Roman enclosure in the modern street plan.