'And Rabirius was with you both on the roof?' asked Cicero. Glancing up from my notes at the old man – his expression vacant, his head trembling slightly – it was impossible to imagine him involved in such an action.

'Oh yes, he was there,' confirmed Isauricus. 'There must have been about thirty of us. Those were the days,' he added, bunching his fingers into a gnarled fist, 'when we still had some juice in us!'

'The crucial point,' said Hortensius wearily – he was younger than his companions and obviously bored of hearing the same old story – 'is not whether Rabirius was there or not. It's the crime with which he is being charged.'

'Which is what? Murder?'

' Perduellio.'

I must confess I had never even heard of it, and Cicero had to spell it out for me. ' Perduellio,' he explained, 'is what the ancients called treason.' He turned to Hortensius. 'Why use such an obsolete law? Why not just prosecute him with treason, pure and simple, and have done with it?'

'Because the sentence for treason is exile, whereas for perduellio it's death – and not by hanging, either.' Hortensius leaned forward to emphasise his words. 'If they find him guilty, Rabirius will be crucified.'

'What is this place?' demanded Rabirius, getting to his feet. 'Where am I?'

Catulus gently pressed him down into his seat. 'Calm yourself, Gaius. We're your friends.'

'But no jury is going to find him guilty,' objected Cicero quietly. 'The poor fellow's clearly lost his brains.'

' Perduellio isn't heard before a jury. That's what's so cunning. It's heard before two judges, specially appointed for the purpose.'

'Appointed by whom?'

'Our new urban praetor, Lentulus Sura.'

Cicero grimaced at the name. Sura was a former consul, a man of great ambition and boundless stupidity, two qualities which in politics often go together.