Scipio, master of Megara, will undertake the blockade of Carthage; he first had a 4.5 km long ditch dug which completely blocked the isthmus and behind which he set up his camp; another ditch, parallel, will protect him on the land side, and finally two perpendicular ditches will lock up his troops in a sort of quadrilateral. Each ditch is protected by a palisade of sharp piles, replaced near the city by a wall 4 m high and 2 wide on which rise towers to watch over the besieged. These gigantic earthworks are carried out fairly quickly by the Roman legionnaires who are very trained in this kind of exercise.
However, if the blockade is airtight on the land side, it is not so on the sea side. in areas that have remained faithful. The Roman boats, as soon as the sea is a little bad, do not manage to thwart all the tricks of the Punic ships. In addition, the Carthaginian commander decides that the food will be shared only between the 30,000 people who defend the city:soldiers and navvies. The rest of the population will manage as they wish so as not to starve. All the poor being mobilized, this remnant includes above all the rich who organize themselves to charter boats to fetch supplies at exorbitant prices.
Scipio then realizes that the only way to starve Carthage is to obstruct its port; a dyke formed of huge blocks of stone is undertaken. It is a gigantic job in which thousands of hands collaborate day and night and which lasts several months. The Carthaginians, always ingenious, decided in response to drill another exit for their port through the surrounding wall, on the eastern coast, and in a place where it would be impossible for Scipio to build another dyke. Men, women and children, all work there, day and night; at the same time an entire fleet is built from all the old materials that can be salvaged in the city. The beams of the houses are themselves used. It is a veritable tour de force attested not only by Appian, but by Strabo and Livy taken up by Florus; 120 ships, triremes and quinqueremes come out of these improvised workshops. Once the canal and the fleet are completed, the perimeter wall is drilled last. As soon as this last operation was carried out, the fleet went out to march proudly before the dismayed Romans:despite their spies, they had not revealed the Carthaginian secret. All the crews of their boats having been requisitioned to build the dam, the latter were empty, and one can wonder why the Punics did not take advantage of their surprise effect. The Roman fleet destroyed, Carthage regained control of the seas and Scipio had only to raise the siege. Certainly, the breach cut in the wall remained narrow to allow the boats to flow back in case of adversity; when three days later it is enlarged, the Carthaginians will come out, this time to fight, but the enemy ships will have regained their numbers and will be ready for battle.