Archaeologists have found fragments of Stirling Castle's 16th Century outer defences.
The discovery was made during work to extend the castle's main shop and ticket office.
Historic Scotland said the find would help establish exactly where the defences stood.
European experts are believed to have been used to apply the latest Italian military engineering techniques at the castle in the 1540s.
They were brought in by Mary of Guise, widow of James V, at a time when intermittent warfare with England made it essential to have fortifications that could protect against heavy artillery in a siege.
The defences are shown in a 17th Century engraving by John Slezer.
Gordon Ewart, of Kirkdale Archaeology - whose team discovered the walling - said: "We knew the defences would have been in this area, but not exactly where because the Slezer engraving, and remaining military plans, are not entirely accurate.
"This is what makes the discovery of physical evidence so important - it helps us identify exactly what existed - and to understand more about what the castle was like in the past."
Much was changed at the castle between 1711-14 when the old defences were demolished during a programme of modernisation.
Further alterations took place when the esplanade was created in the early 19th Century.
Peter Yeoman, from Historic Scotland, said the discovery gave a "tantalising glimpse" of the fortifications created for Mary of Guise and paid for by the French king Henri II.
Mr Yeoman said they were probably designed by the Italian engineer Signor Ubaldini, who was working on a similar defensive spur at the time at Edinburgh Castle.
He added: "They are of great interest because they were early examples of a changing approach to military engineering, and among the most advanced in the whole of the British isles."