Houses were built within insulae, parcels of land delimited by roads of varying sizes and orientations. At first, dwellings were only found along the city's primary and secondary roads. Later, they were constructed in the interior of these parcels. Areas or passageways between the dwellings that remained unbuilt were lined with small pebbles. An internal circulation system developed between the densely packed houses: the ambitus-a narrow alleyway often paved with limestone slabs.