Augusta Praetoria was laid out on a standard Roman military grid, and that grid has survived as the street plan of Aosta's historic centre for over two thousand years. The cardo maximus (north–south axis) and decumanus maximus (east–west axis) are still the main streets of the old town. The insulae — the rectangular blocks between the streets — remain largely intact in their proportions, even though the buildings on them have been rebuilt many times over.
The walls themselves, built of local granite and concrete, enclosed the entire colony in a rectangle that can still be walked in under an hour. Substantial sections survive, particularly on the north and west sides. The Tour du Lépreux (Leper's Tower) at the northwest corner is the best-preserved of the original wall towers, standing nearly to full height with its Roman core intact. It takes its name from the medieval practice of confining lepers in the tower's upper chamber — a grim later history for a Roman fortification. Walking the perimeter of the walls gives a clearer sense of the original scale of the city than any individual monument.