The Romans built ports at Arno and Mugnone to create advantageous transport positions; old Florence was on the Via Cassia, forming a wedge controlling the end of the Apennine valley of Arno and the beginning of the plane that led to the sea in the direction of Pisa. In AD 123, a bridge was constructed over the Arno. Buildings began to accumulate around the Roman military camp, including an aqueduct (from Monte Morello), a forum (in today's Piazza della Repubblica), spas, the Roman Theatre of Florence, and the Roman Amphitheatre of Florence, while the surrounding land was organized by centuriation. A nearby river port allowed trade up to Pisa. The outlines of the Roman city are still recognizable in the city's plan, notably the city walls.