He was right. It was a trick Brigonius had learned from Tullio the prefect, who was becoming a brusque sort of friend. ‘All soldiers are lazy blighters,’ Tullio would growl in his thick Germanic-tinged Latin. ‘It’s in the blood. The only thing to do is to sneak up on them from a direction they don’t expect, at a time they don’t expect it. The small hours is my favourite time. You should see the whores and catamites run, like pink rats!’

Brigonius asked Matto, ‘Am I that predictable?’

‘You’ll have to find some new spying-points. Here’s the latest tally.’ Matto handed Brigonius a fold-out notebook of taped-together wood leaves.

Brigonius looked over the figures; at first glance they seemed in order. ‘Any problems?’

‘None but the Romans,’ Matto growled. ‘The numbers add up in their neat columns. But they are stealing our stone, cousin.’

‘No, they aren’t,’ Brigonius said patiently. They had had this argument many times before.

‘The prices they are paying are ruinous,’ Matto insisted.

So they were, even compared to the prices Brigonius had been able to extract from the soldiers at Vindolanda before the coming of Hadrian and his plan. But at least they did pay, when they could have just taken the stone. Brigonius thought he understood. For one thing by paying rather than stealing, even a pittance, the Romans kept their own consciences clean; they might be hypocrites, but they preferred to run their affairs according to the rule of law - that is, their law.

And there was a subtler purpose. When he had come to work for Brigonius Matto had had to learn Latin, and to write and tally well enough to keep adequate records. Matto may not have realised it - and Brigonius wasn’t going to point it out - but by being forced to deal with the army he was becoming, little by little, literate and numerate, and locked into the Romans’ economy.

The Roman army wasn’t just a tool of conquest. The largest organisation in the world, boasting three hundred thousand men deployed from the Tinea to the Euphrates, everywhere it worked it used Latin and paid in the imperial coin. The army was a source of Romanness, its walls and forts a stony wave of acculturation.

Anyhow, such was the quantity of stone being extracted to build the Wall that even at the Romans’ ‘ruinous’ prices Brigonius and his family were growing quietly rich.

Matto was recounting a long and complicated anecdote about the behaviour of a particularly obnoxious decurion in the quarry. Brigonius cut him off. ‘You’re even more sour than usual today, cousin. Something’s been twisting your crab-apples. What’s up?’

‘Everybody’s stirred up,’ Matto said. ‘It’s the tax census.’

‘Another one?’