English civil war story: writing a treachery and violence scene

This post looks at writing a scene for a story about the English civil war that involves treachery and violence between the intelligence agencies of the Royalist forces and the Parliamentary forces

I’m pretty convinced that each army in the English civil war had its own intelligence agency (although they would not have been called by that name), and it’s reasonable to suppose that these intelligence agencies would have resorted to treachery and violence.

In this scene, captain Cavenham, a Royalist intelligence officer, has just been briefed for a mission by a senior Royalist intelligence officer in a London coffee house.
He trusts the senior intelligence officer and believes that the mission is secret.
But both are errors of judgement.
Here’s the scene:
‘Outside the coffee house he hailed a cab.
“Spare a copper, sir, for a soldier a the King.” A man stood on a single leg, supporting what would have been his other leg by resting on a crutch. On the missing leg his breeches were tucked above the knee, and Cavenham guessed that a cannon ball had torn away his lower leg.
“You fought for the King?”
“A course, sir,” he said indignantly.
Cavenham studied him expressionlessly. London – the people of London, overwhelmingly supported the Parliamentary cause. It was passing strange that someone should declare their support for the King.
It was even more passing strange that this someone should be a soldier who had fought for the King.
Or who claimed thus.
Cavenham turned to his cab again – and paused, surprised. It seemed different, the coachwork – the painted lines that swept along the sides of the carriage – seemed different somehow.
He glanced up at the driver, who, high on his seat, was looking impassively ahead.
Something was wrong. This wasn’t the cab he had hailed. Someone – he knew not who – had substituted it for his cab. If so, what had happened to the cab he had hailed? Cab drivers didn’t just disappear and allow a rival to steal their fare.
He shrugged his shoulders as if puzzled and placed a foot on the step of the cab.
Now.
Now was the time, when the conspirators -whoever they were – were lulled into a sense of security, when they believed that he would behave predictably, when they believed that he would turn his back on them and get into the cab.
Suddenly he spun around and drew his sword – but he was too late.
The one-legged soldier – now on both feet – nimbly swung the cudgel he had concealed and Cavenham’s consciousness disintegrated into a world of exploding stars and distant voices.
“God damn your King, captain,” grunted the ‘one-legged soldier’ who had ‘fought for the King.’
“For God and Parliament!”