ARCHAEOLOGISTS have unearthed the remains of a medieval tower at Edinburgh Castle thought to have been lost forever.
Fragments of Constable's Tower, which was destroyed by Elizabeth I's army during a siege, were found during excavation work for the attraction's new visitor centre.
A team of experts found a drain beneath the surface just inside the Castle's main portcullis gate, where a new timber kiosk selling audio tours is to be built.
They were amazed to find part of the disused drainpipe had been from a three-foot long piece of ornately carved masonry. Archeologists now believe it originally came from the lost Constable's Tower, which stood from the 14th century to the "Lang Siege" of 1581-73.
Historic Scotland archeologist Peter Yeoman said it was a "very significant" find and added that it could be the only remaining fragment of the tower.
He said: "This has the potential to tell us much more about this tower. There are a few illustrations from the 1500s which feature the Constable's Tower, so we always knew roughly where it was although none of it had been found. We thought it had all been demolished and thrown away until we saw this piece of pipe.
"When I first saw it, I could see straight away that this was a fragment of a very fine glazed window probably of 15th or 16th century date. It must have been lying around as a piece of rubble and ended up being built into this later drain. Putting two-and-two together, I realised that the only building it might have come from in this vicinity was the lost Constable's Tower.
"Such a large window - at least 5ft high - might have graced the constable's private apartments, from which he could have looked out on to the coming and goings of the royal court in the nearby royal palace buildings. More warlike, smaller windows would have looked out to the town below."
The Constable's Tower was built during the reign of Robert II and was the home of the castle's constable - a powerful position appointed by the king to watch over and maintain the fortress in his absence. It was finally destroyed in 1573, after an epic siege which saw the castle garrison led by Sir William Kirkcaldy of Grange hold out against Scottish forces loyal to the infant James VI.
After Elizabeth I sent troops and cannons to bombard the castle, the tower was destroyed and later replaced with its current portcullis gate facade.
Archeologists now plan to study the piece of masonry further to build up an image of what the tower window would have looked like.
However, the artefact may be put on show within the Castle once the analysis is completed.
Barbara Smith, executive manager of Edinburgh Castle, said: "Thanks to our archaeologist's nimble eye, another significant piece of Edinburgh Castle's complex history has been found and we are absolutely thrilled.
"Each discovery gives us a fascinating glimpse into the past, helping us to understand more fully the Castle's role through the ages. It is an exciting find, proving that we are not even close to realising the full extent of buried treasure at Edinburgh Castle."
The archaeological probe is part of a 2.7 million project to build a new visitor reception area. The project will include a new ticket office and terrace, effectively transforming the visitors' view of the Castle's main entrance by removing the existing ticket office from the Esplanade.