‘Little mice?’ Arrian was frowning, then realised he had spoken his thought out loud. Musculi were sheds, the frames mounted on wheels and covered with hides as protection. They allowed soldiers to get closer to the enemy in spite of missiles so that they could work. ‘Sorry,’ the tribune said, ‘I’m a mere Greek, and I’d never thought about it before. Why are they called that?’

‘Because they creep up on the enemy,’ Rufus suggested. ‘You know the way women are frightened mice will run up their legs.’

‘To be honest, no,’ Cerialis said.

Crispinus raised his hand as a gesture for the others to be silent. ‘Such matters are surely of interest, but let us save them for another day. We have done a lot, and must keep up a steady pace. You are content with the allocation of work, primus?’

‘Yes,’ Caecilius grunted and nudged Rufus.

‘Each cohort divided into four teams,’ the young centurion explained. ‘Deducting men assigned to the engines, to guard duty, administration and other tasks, that gives some sixty to seventy in each team. Each will alternate between a day on outpost duty, a day with the work parties, a day in reserve and another day half as work parties and half standing guard during the night. Cavalry get a day on outposts, a day in reserve, a day with the horses, and a day as camp guards. None of that includes the cohortes equitatae, who will mainly be concerned with foraging and escorting the supply columns to the river and back. Ideally it’s two days there, two days back, and a column sets out every fourth day.’

‘That sounds admirable,’ Crispinus told them. ‘I leave it to you, primus, to assign each group with its task. So, show me the main projects.’

Caecilius stood up. ‘By your leave, sir.’ He had a wooden ruler of the sort engineers used, and pointed at the map. ‘The gates on both sides are death traps. The gates themselves can be burned or smashed, but the towers and walls around them are the strongest parts of the defences, and it would take years to knock them down. If we smash the gates any assaulting party is trapped in a courtyard, with no way out and no real way of getting a battering ram against the inner gate, so they die where they stand.’

‘So we don’t go that way,’ Crispinus said.

‘No, sir, we don’t. These walls are sandstone. Not the strongest. It’s harder than mud brick, but crumbles more easily if you can prise it apart. If you look at this plan, you can see that they have towers at regular intervals along the wall.’ The officers rose, even though they had all seen the real defences plenty of times. ‘Those towers stick out in front of the walls – project is the fancy term technical engineers like Rufus here use.’ Caecilius tussled his colleague’s hair as if he was a small boy. ‘Whatever you call them it means that they can shoot sideways along the line of the wall. So to make their life as hard as possible, we go for the corners. Here in the north west.’ He pointed with the ruler. ‘And here in the south east, over on the prefect’s side. There isn’t a real corner, but we go as close as we dare to the tower where the walls curve off on the far side.