It was built for an invasion that never came.

But the defences of the historic Derby Fort at Langness proved no match for a determined intruder who forced their way inside, damaging stonework and pushing an ancient cannon off its plinth.

Andrew Johnson, inspector of ancient monuments at Manx National Heritage, said: ’We are appalled and saddened. There is a lot of damage. To go to such lengths seems selfish and thoughtless.’

He said the cannon weighs more than 100 kilos and the consequences could have been dire if it had landed on the intruder’s foot.

Mr Johnson said: ’They are important artefacts in their own right but if they are mishandled they could be damaged and equally they are of such a weight they could cause a nasty injury.’

The fort is not open to the public and its entrance is guarded by a solid, locked, gate.

Explaining how its defences had been breached, Mr Johnson said: ’Unfortunately, somebody has given the lock mechanism on the gate a real pasting. They have tried really quite hard to force an entry but were not successful.

’They have then got in through one of the gun embrasures. They have loosened one of the many bars in the gun ports and in doing so damaged the masonry.’

Once inside, the intruder caused a lot of damage, dislodging and levering out masonry as well as pushing a cannon off its plinth.

There are indications that the vandal may have camped in the fort overnight.

’There was a sheet of roofing felt in one area,’ explained Mr Johnson. ’It’s possible they were staying there overnight, or they have been just sheltering from the weather.

’Give the location of the fort it is possible to get inside unseen and remain unseen once inside.’

Police inquiries are continuing in a bid to catch the vandal. Damaging an ancient monument is an offence under law punishable with a fine of up to £5,000 and even prison. There is also provision in law for the offender to meet the cost of repairs.

’We are liaising with police,’ said Mr Johnson. ’This isn’t something that gets sorted in a few days. It will need a quite an substantial investment of time and taxpayers’ money.’

But how can MNH protect the building against future incursions? Will they have to resort to fortifying it with modern-day defences such as CCTV?

’It might come to that,’ admitted Mr Johnson. ’We want to make sure the building is safe. We do open it on special occasions.

’This is part of our heritage, it’s something we should be proud of. It might be a relatively small fort but it is nevertheless an important historical structure in terms of what it represents.’

The fort was built in the 1530s as part of Henry VIII’s coastal defences. It did not see any action but it was clearly thought of as a worthwhile fortification more than century later when many alterations were made by the 7th Earl of Derby during the Civil War.