A town plan, drawn around 1539, has been found among thousands of documents in British Library archives.

The map shows the coastal town's fortifications and reveals that a cannon was installed within the castle walls.

The permanent presence of the cannon, a fairly new military advance at the time, suggests that Scarborough was a key stronghold against invaders from France and Spain.

The map came to light when the British Library was preparing an exhibition on Henry VIII to mark the 500th anniversary of his accession to the throne.

English Heritage events manager Jon Hogan said: "It was a chance find – there must be thousands of documents that are not on public display, but this one really does tell us a lot more about how Henry VIII perceived Scarborough as a northern stronghold.

"At this time, maps and plans of locations were relatively rare, and would only be done to assess a site's importance."

He said the fact that the cannon was on the map suggested it was a permanent fixture at a time when Henry was known to be diverting his military resources to fortifications on the south coast.

Visitors to Scarborough Castle will get a chance to see – and hear – how the cannons would have sounded at a Tudor event this weekend.

Mr Hogan said: "The sound of the cannons will reverberate around the North and South bays as the gunners engage in a spot of target practice - although without real cannon balls of course!"