It is on the soil of the present-day Austrian states of Upper and Lower Austria. It ran along the Danube from Passau/"Boiodurum" to Zeiselmauer/"Cannabiaca". This is also a "ripa" (river border), which was guarded by a loose chain of cohort forts. The main road on the Norican Limes was the "via iuxta amnem Danuvium". The initially simple wood and earth structures were systematically converted under Emperor Hadrian into stone encampments. During the 4th century, they were brought once more up to date and massively reinforced. Between the camps, in strategic places or good points of observation, were watchtowers or signal towers and, in the Late Antiquity, "burgi". In the middle section, between the camps of "Favianis" and "Melk", watchtowers were built only sporadically. Here the narrow valley of the Wachau, with its densely forested escarpments, made access to the riverbank more difficult, providing some defensive function. Every camp had its own river port or landing stage and a storage area because the Danube was not only border zone, but also the most important transport and trade route in the region. Over time civilian settlements or "vici" were established immediately next to the camps. In the immediate hinterland of the "limes", walled towns or "municipia" were founded - for example, "Aelium Cetium" or "Ovilava" (Wels). They were the administrative or commercial centres of the region. In late antiquity, the Norican area was divided into two parts ("pars inferior" and "pars superior"). It is probable that a second, defensive line was created to the rear ("Castrum Locus Felicis").