IT was one of the bloodiest episodes in Scottish history and saw the brief introduction of a republic after a brutal civil war that overthrew the monarchy.

Oliver Cromwell and his New Model Army invaded Scotland in 1650 after the execution of Charles I and the country was eventually made part of the English Commonwealth.

Independence was finally restored in 1660 when Charles II was crowned following Cromwell’s death two years earlier.

Now a fascinating relic of those turbulent times has been uncovered in the Outer Hebrides and has forced harbour bosses to temporarily halt an expansion of the port after the wall of a medieval castle was discovered at the site.

Experts believe the structure, more than 2.5 metres thick and 1.5 metres tall, is part of a medieval fort built by Oliver Cromwell’s army in the 17th century and occupied by a garrison of English soldiers for around a decade.

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They say the find has the potential to provide evidence from early periods of the town.

Stornoway Port Authority has now postponed its future expansion plans to help preserve the piece of the town’s history.

The authority has delayed a project to extend its office at Amity House on Esplanade Quay, close to the ancient centre of Stornoway, after archaeological investigations revealed remnants of a substantial and historic stone wall on the site.

Alex Macleod, Stornoway Port Authority’s chief executive, said: “This is a significant, historic find and an integral part of the town’s past. We are keen to preserve this medieval discovery for future generations.

“As the master plan progresses, the port will need additional office space. However, our current priority is to advance projects, including the new marina and deep-water port, that will stimulate new economic growth for our community which themselves will also be part of Stornoway’s story in time.”

Evidence of such a building had been discovered while digging for drains adjacent to Amity House in 1919 as mentioned in WC Mackenzie’s “Book of the Lews: The story of a Hebridean Isle” but no further investigation had been done until now.

Mary Peteranna, of AOC Archaeology, said: “We have uncovered one section of a substantial wall surviving up to 1.5 metres high.

“The wall face is very well-built and comprised a battered, or slightly sloping outer face; and this, together with its breadth of about two metres, tells us that it’s not just a building wall.

“The structure was built for a more substantial purpose, and we believe it formed part of the Cromwellian defensive rampart.”

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The site is located about 100 metres north of Stornoway’s medieval castle, the site of which has been shown clearly on historical mapping up to the mid-1800s and on top of which No1 Pier was constructed.

Although these same early maps did not depict the wall uncovered on the dig site, a rare sketch plan from 1653 in the National Library of Scotland provides details of the Cromwellian fort in Stornoway.

Cromwell, who garrisoned forts across Scotland to bring the country under control, took an interest in the port town after an uprising led by Kenneth Mor Mackenzie in favour of the Royalists.

The fort occupied the Stornoway peninsula, including the former church of St Lennan which became a dormitory for soldiers.

The 1653 plan shows a manor house, storehouses, two to three storeys high, a brewing-house and draw wells.

While the fort would have been protected by water on seaward sides, defensive ditches were on the landward sides.

The office extension was proposed to cater for new staff as the port authority develops its 20-year Master Plan which will bring substantial economic benefits to the islands.

Alternative accommodation has now been identified.

Stornoway Historical Society chairman Malcolm Macdonald said: “We are delighted that the recent discovery of walls belonging to the Cromwellian fort, establishing the precise location, permits historians to mark the site for posterity.”