The interventions planned in 2006 for the study of the re-modernisation of piazza Roncas, prior to the actualisation of a specific project, were born out of an investigation of the archaeological remains of the _Porta Principalis Sinistra_, the north gate of the Roman and later of the medieval town. The first campaign of extensive stratigraphic excavation led to the investigation of the eastern tower, in particular the relationship between this structure and the surrounding buildings, highlighting the complex topography of this important part of the town. The trench reached the foundations of the tower and brought to light some stretches of its monumental exterior in _opus quadratum_. The structure was better defined by the integration of an internal wall into the schematic plan made by D’Andrade in 1894. Unfortunately the tower has been substantially altered during the course of the modern era and this has complicated the stratigraphic interpretation of the complex. The area between the east tower and a large rectangular building, built during the course of the 1st century A.D. and situated circa 6 m south of it, was in a much better state of preservation. Between the two buildings a beaten gravel surface was identified, probably an open air passage which may have been a side street leading off of the _Cardo maximus_. On its surface were linear cuts on an east-west alignment running parallel to the tower and the south building. The repetition of these cuts in the surface of the subsequent phase suggests that they were the housings for basalt elements forming kerb stones. Therefore, this would have been a secondary road, mainly for pedestrian use, with 1.8 m wide sidewalks flanking a central carriageway that was circa 2 m wide. The southern limit of the side road was a large rectangular room (east-west width almost 13 m, length over 14 m) which developed south of the tower along the last urban stretch of the via publica. Its north-eastern corner, visible below the Archaeological Museum, was already known. For the construction of the building’s southern edge substantial levelling had been undertaken of the basal deposits which consented the creation of a pavement at a similar height to that of the Cardus maximus. Excavation of the building is still in progress.