The colony of Augusta Praetoria was founded in 25 B.C. after the war against the Salassi, the inhabitants of the territory between Eporedia (Ivrea) and the mountain passes of the Alpis Poenina (Gran S. Bernardo) and the Alpis Graia (Piccolo S. Bernardo). The colony formed part of a project to develop the alpine road system. In fact, in the centre of the city, the road from Eporedia forked to head towards the Gran S. Bernardo via the Porta Principalis Sinistra, in the northern city wall, and the Piccolo S. Bernardo via the Porta Decumana to the west. A first investigation on the site of the Porta Decumana (an urban area of continuous occupation) in the modern via E. Aubert, was undertaken between 1988 and 1991 during construction of the new regional library. Further investigation took place during work on the tunnel for services in 1999-2001. The excavation campaign carried out between the end of 2004 and spring 2005 involved a series of careful interventions in the area occupied by ancient structures. Excavation enabled the reconstruction of the gate's form in the mid Imperial period: it had three arches, the central one being the widest, opening in a double curtain wall and was flanked by a tower on either side. The north tower is conserved to a considerable height, whilst the south tower is almost completely razed to the foundations and is visible in the library basement. Large stretches of the decumanus maximus, of Tiberian date, paved with slabs of bardiglio also came to light. Above this lay an early medieval paving of which a wide stretch is conserved. During the 4th to 5th centuries A.D. the gate's northern opening was blocked and the width of the central archway reduced, an alteration perhaps dictated by defensive needs. The partial closure of the gate favoured the development of housing in this area that was no longer transitable. Important data was recovered regarding an earlier phase of the Porta Decumana, the structure of which is being studied at present. (Antonina Maria Cavallaro)