Scipio nodded. ‘The dynamic of the war against the Celtiberians has changed. The Arevaci are the largest tribe, and their main oppidum is Numantia. Take Numantia, and the other oppida of that tribe might fall to you without a fight, and the war would be over.’
‘Is that Lucullus’ plan?’ Fabius asked.
Scipio’s face was impassive. ‘He has only one legion, freshly raised and inexperienced. He intends to win enough sieges for a triumph, and then to leave. But by coming to Spain with no more than personal glory in mind he has set in train a war with Rome that will not be extinguished until Numantia is taken, perhaps years from now. That’s what Ennius and I have been war-gaming.’
‘What would you do?’ Fabius asked.
Ennius pointed with his stick. ‘This is the river Durius. I’d build towers on either side of the river, in two places five hundred feet apart. The towers on the near side of the river would be close enough for archers to rain down arrows inside the oppidum. I’d circumvallate the oppidum with a deep ditch and rampart, and double it outside the main entrances where a strong force sallying forth might overwhelm a single ditch system.’
Scipio grinned at him. ‘Spoken like a true engineer. You’d build another set of walls around Rome if you had the chance.’
‘That’s not a joke. The city is getting too big for the Servian walls. They’re over two hundred years old now. And the more wooden tenement houses that are crammed inside the walls, the more likely there is to be a devastating fire.’
‘Polybius and one of his scientist friends from Alexandria did a mathematical calculation about city walls,’ Scipio said. ‘They established that unless you have a population even more densely crowded than the population of Rome, living in tenements that would have to be eight or ten storeys high, you simply wouldn’t have enough manpower in a city to defend its outer limits.’
Ennius nodded. ‘City walls are only really ever for show.’
‘You need defence in depth, a smaller area of fortification to fall back on. That’s what the Celtiberians did here at Intercatia a week ago.’
‘Do you remember Polybius taking us to Athens and showing us the Acropolis? That’s something the Greeks have got right, and we haven’t.’
‘Because the Roman spirit is offensive, not defensive. But the Celtiberians, like the Greeks, are generally inward looking; it’s unusual for them to expand beyond their borders and to take over adjoining oppida. Rome, by contrast, has been outward looking for centuries now, devouring surrounding tribes and then the city-states of the Greeks and the Carthaginians, forever expanding.’
Ennius gave him a wry look. ‘Yes, and see what happens when invaders do reach Rome: the Gauls two and a half centuries ago, and very nearly Hannibal in our grandfathers’ time. The Capitoline Hill where people took refuge from the Gauls was easily overwhelmed, and remains unfortified. One day Rome will reach the limits of its expansion and will suffer from the same weakness revealed by Polybius’ calculation, of not having enough manpower to defend the frontiers. Yet great efforts will be expended to fortify the frontiers at the expense of Rome itself, which will remain vulnerable and will fall.’
Scipio grunted. ‘The Celtiberians regard their oppida as refuges, as do the Gauls,’ he said. ‘The lower courses of their walls are built of stone, the upper structure of wood with thatched roofs, vulnerable to fire. That is their greatest defensive weakness. They knew nothing of siege engines when their walls were designed.’
Ennius nodded. ‘I would bring up batteries of ballistas and catapults, for solid shot and fireballs.’
Scipio pursed his lips. ‘The river is still the weak point.’
Ennius stared for a moment, and then traced a line across the furrow between the two stones. ‘What about this. You attach a thick cable between the towers, tensed so that it lies on the surface of the water. You twist the cable around sections of tree trunks, so that they form a boom. Then there is no way that boats could be dispatched from the oppidum to reach safety.’
Fabius looked at him. ‘I have a suggestion.’
‘Speak your mind.’
‘Have you ever been to the chariot races in the Circus Maximus when they attach blades to the wheels?’
‘A great spectacle, total carnage,’ Ennius said. ‘It’s not just what the blades do to the chariots when they lock together, but to the charioteers who fall within them.’
‘What’s your point, Fabius?’ Scipio said. ‘Numantia’s a long way from the Circus Maximus, and chariots would just bog down in the mud out here.’
‘Not chariots, Scipio, but those floating logs. A week after we arrived in Spain I went with a reconnaissance patrol to Numantia, to size up the defences. Now that I know your model is meant to represent the oppidum, I recognize the lie of the river. At those points where you’ve put the towers it flows particularly fast, being narrower, especially when it’s bloated with the rains that seem to fall all the time here. Instead of seeing that weather as an impediment, we could turn it to our advantage. Paddles affixed like the spokes of a wheel at either end of those logs would make them spin around with the current.’
‘I’ve got you,’ Ennius said enthusiastically. ‘Attach blades jutting outwards along the length of the logs, and they would scythe away like the wheels of a chariot. Not only would boats be unable to get through, but neither would swimmers.’
Fabius took the stick from Scipio and traced two lines across the furrow. ‘The river is nearly fordable at these points. Place your towers and the log booms there, and the blades would nearly brush the riverbed. Swimmers would be unable to dive beneath.’
Ennius nodded, staring at the mud. ‘A brilliant suggestion, Fabius. That’s one for Polybius’ textbook. If the Intercatians continue to tax our patience and hold out longer, I will keep my fabri occupied by having them build an experimental boom on the river here to see how it works.’
Scipio slapped Fabius on the shoulder. ‘We’ll make a general of you yet.’
‘Centurion will do, Scipio. One day, when I’ve earned it.’
Ennius peered at Scipio. ‘So much for our siege works. How would you dispose your men?’
‘One third for the assault force, one third in reserve. One third of the reserve to move up and man the enemy walls once the assault force has moved through the breaches made by artillery, including all available archers and slingers. The forward line of the reserve to include fabri ready to spring forward and provide scaling ladders and demolition teams if called for. The remaining third of the force to comprise ballista and catapult crews, the heavy cavalry to repel any sally from the enemy and a light cavalry force to hunt down any who would escape from the oppidum to seek aid.’
Ennius grinned at him. ‘Now that’s straight out of the textbook.’
‘I’ve had plenty of time to prepare. When I haven’t been hunting and training, I’ve been war-gaming. The law courts and the debating chamber only take up a few mornings every week. They’ve knocked down the old Gladiator School where we held the academy, but Fabius and I managed to salvage the diorama table where we studied battles. Whenever Polybius and any of the others are around we get together in a room I’ve had specially added to my house on the Palatine and recreate the great battles of the past, changing the variables to try to alter the outcome, just as we were taught to do. We must have done Zama fifty times, Cannae about the same. But my special fascination has always been sieges.’