As well as manning the walls of Carthage, the Carthaginians formed a field army under Hasdrubal, which was based to the south. The Roman army moved to lay siege to Carthage, but its walls were so strong and its citizen-militia so determined it was unable to make any impact, while the Carthaginians struck back effectively. Their army raided the Roman lines of communication, and in 148BC Carthaginian fire ships destroyed many Roman vessels. The main Roman camp was in a swamp, which caused an outbreak of disease during the summer. The Romans moved their camp, and their ships, further awayso they were now more blockading than closely besieging the city. The war dragged on into 147BC.